Comparative characterization of Arabidopsis Subfamily III β-galactosidases

Dashzeveg Gantulga

Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State

University in partial fulfillment of the requirement for the degree of

Doctor of Philosophy

In

Biological Sciences

Dr. Brenda S.J. Winkel Committee Chair

Dr. David R. Bevan Committee member

Dr. Richard A. Walker Committee member

Dr. Zhaomin Yang Committee member

December 5, 2008

Blacksburg, Virginia

Keywords: Arabidopsis, β-galactosidase, cell wall

Comparative characterization of Arabidopsis Subfamily III β-galactosidases

Dashzeveg Gantulga

Abstract

The Arabidopsis genome encodes 17 putative β−galactosidases belonging to

Glycosyl Hydrolase (GH) family 35, which have been classified into seven subfamilies

based on sequence homology. The largest of these, Subfamily III, consists of six genes,

Gal-1 (At3g13750), Gal-2 (At3g52840), Gal-3 (At4g36360), Gal-4 (At5g56870), Gal-5

(At1g45130), and Gal-12 (At4g26140) that share 60-81% sequence identity at the amino

acid level. All six proteins have a signal peptide that may target them to the cell exterior.

We report purification and biochemical characterization of all six members of

Subfamily III, each expressed as a recombinant protein in Pichia pastoris and one also in

native form, purified from Arabidopsis leaves, with a special emphasis on substrate

specificities. Organ specific expression of the six Gal genes was examined by analysis of

the microarray databases and by semi-quantitative RT-PCR. The relative abundance and

size of the Gal-1, Gal-2, Gal-5, and Gal-12 proteins was studied by immunoblotting using

isoform-specific anti-peptide antibodies. The protein expression patterns of the Gal genes

were generally consistent with microarray and RT-PCR data, though some discrepancies

were observed suggesting distinct mechanisms of regulation for transcription and

translation. Localization of total β-galactosidase activity was visualized using the

substrate, 5-bromo-4-chloro-3-indolyl-β-D-galatopyranoside (X-Gal), to stain whole

plants. Subcellular localization of the four isoforms examined by immuno-dotblotting

and western blotting showed that Gal-1, Gal-2, Gal-5 and Gal-12 are present in apoplastic

and cell wall bound protein extracts. Immuno-EM analysis of Gal-1 and Gal-12 showed

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that these proteins are localized in the cell walls of vascular and epidermal tissues in

mature root. Taken together, the biochemical properties, expression patterns, and

subcellular localization of these isozymes indicate that the Subfamily III β-galactosidases

all have potential functions in restructuring the cell wall during plant growth and

development.

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Acknowledgement

First of all, I would like to thank Dr. Winkel for everything she had done for me. She has

encouraged me throughout the graduate study and helped me reach the point where I am

now. I would like to thank my committee members, Dr. Walker, Dr. Bevan, and Dr. Yang

for valuable advices and critical comments over the course of my study here at Virginia

Tech. I would like to thank members of the Esen’s lab, past and present, for providing me

help on various levels throughout the course of this degree program. I wish to extend my

thanks to many people from Department of Biological Science and other departments

who have helped me technical side of the research. I could not have come this far without

the tremendous support and constant love of my husband and our wonderful children.

This was a great adventure and certainly life changing experience. Thank you. I am also

grateful to my parents, brothers, sister and brother and sisters in-law for their love and

support.

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LIST OF ABBREVIATIONS

6BNG 6-bromo-naphthyl-β-D-galactopyranoside

CM Accell plus carboxymethyl

ConA Conconavalin A

Gal β-galactosidase

GH Glycosyl hydrolase

IPTG isopropyl-β-D-galactopyranoside

BMGY Buffered complex glycerol media

BMM(H) Buffered minimal methanol media (supplied with histidine)

MALDI-TOF Matrix-assisted laser-desorption-ionization–time-offlight

4-MUGal 4-methylumbellriferyl-β-D-galactopyranoside

nGal-2 native Gal-2

N-link Asparagine linked glycosylation site

oNPGal ortho-Nitrophenyl-β-D-galactopyranoside

PBS phosphate buffered saline

PBST phosphate buffered saline with tween-20

TBST tris buffered saline with tween-20

TBSTh tris buffered slaine with tween and high salt

pNPAra para-Nitrophenyl-α-L-arabinopyranoside

pNPGal para-Nitrophenyl-β-D-galactopyranoside

pNPGlc para-Nitrophenyl-β-D-glucopyranoside

pNPFuc para-Nitrophenyl-β-D-fucopyranoside

pNPMan para-Nitrophenyl-β-D-mannopyranoside

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pNPXyl para-Nitrophenyl-β-D-xylofuranoside

TBG Tomato β-galactosidase

TEM Transmitting electron microscopy

X-Gal 5-bromo-4-chloro-3-indolyl-β-D-galatopyranoside

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Table of Contents

Chapter 1. Literature Review 1

1.1 Classification of Gals and their catalytic activity 2

1.2 β-galactosidases in different organisms 5

1.3 Arabidopsis thaliana β-galactosidases 9

References 16

Chapter 2. The Arabidopsis At1g45130 and At3g52840 genes encode β-

galactosidases with activity toward cell wall polysaccharides. 22

Abstract 23

1. Introduction 24

2. Results and Discussion 26

2.1 In silico analysis of Gal-5 and Gal-2 26

2.2 Microarray data for expression of At1g45130 (Gal-5) and

At3g52840 (Gal-2) transcripts 27

2.3 Expression of Gal-5 and Gal-2 in Arabidopsis:

western blotting studies using peptide-specific antibodies 30

2.4 Cell wall localization of Gal-5 and Gal-2 proteins: dot-blotting 33

2.5 Expression of Gal-5 and Gal-2 in P. pastoris and purification 35

2.6 Properties of recombinant Gal-5 and Gal-2 37

2.7 Aglycone, glycone and natural substrate specificity 38

Conclusions 44

3. Experimentals 46

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Aknowledgement 53

References 54

Chapter 3. Comparative characterization of the Arabidopsis

Subfamily III β-galactosidases 60

Abstract 61

1. Introduction 62

2. Results and Discussion 64

2.1 In silico characterization of the Arabidopsis

Subfamily III β-galactosidases 64

2.2 Expression of β-galactosidases in Arabidopsis 66

2.3 Expression and purification of recombinant

Gal-1, Gal-3, Gal-4 and Gal-12 in P. pastoris 70

2.4 Properties of recombinant β-galactosidases 71

2.5 Isolation and characterization of native Gal-2 protein 79

2.6 Localization of β-galactosidase activity in whole tissues 82

2.7 Organ specific expression of Gal-1 and Gal-12:

Immunoblotting using anti-peptide antibodies 82

2.8 Immunolocalization of Gal-1 and Gal-12 86

2.9 Conclusions 88

3. Experimentals 90

Aknowledgement 96

References 97

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Chapter 4. Conclusion 103

Conclusion 104

References 107

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Summary of Figures Chapter 1

Figure 1: Retaining mechanism of enzymatic hydrolysis of the glycosidic bond (Davies and Henrissat, 1995) 4

Figure 2: Phylogenetic analysis of Arabidopsis thaliana β-galactosidases 10

Figure 3: A model of plant cell wall structure (Cosgrove, 2005) 14

Chapter 2

Figure 1: Phylogenetic tree of selected plant β-galactosidases 28

Figure 2: Organ-specific expression of Gal-5 and Gal-2 32

Figure 3: Localization of Gal-5 and Gal-2 in cell wall

fractions by dot blotting 34

Figure 4: SDS-PAGE, western blot, and zymogram of

purified Gal-5 and Gal-2 36

Figure 5: Linkage specificity of Gal-5 and Gal-2 42

Chapter 3

Figure 1: Phylogenetic relationships among the

Arabidopsis thaliana β-galactosidases 65

Figure 2: Expression profile of the Subfamily III

β-galactosidases from the public microarray data 67

Figure 3: Semiquantitative RT-PCR analysis of

Subfamily III gene expression 69

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Figure 4: MALDI-TOF spectrum, SDS-PAGE,

and native PAGE of purified native Gal-2 80

Figure 5: Localization of β-galactosidase activity in

Arabidopsis whole tissues 83

Figure 6: Immunoblot analysis of organ-specific

expression of Gal-12 and Gal-1 84

Figure 7: Subcellular localization of Gal-1 and Gal-12 87

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Summary of Tables Chapter 2

Table 1. Expression profile of Gal-5 and Gal-2 using

microarray data 29

Table 2. Aglycone specificities of Gal-5 and Gal-2 39

Table 3. Sugar specificities of Gal-5 and Gal-2 40

Table 4. Natural substrate specificities of Gal-5 and Gal-2 45

Chapter 3

Table 1. Kinetic parameters 73

Table 2. Sugar specificities of Gal-1, Gal-3, Gal-4 and Gal-12 74

Table 3. Inhibition coefficients 76

Table 4. Natural substrate specificities 77

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Chapter 1

Literature Review

Figure 1 reprinted from Structure 3, Davies, G., and Henrissat, B., 1995a. “Structures and

mechanisms of glycosyl hydrolases,” pages 853-859, with permission from Cell Press.

Figure 3 reprinted from Nature Review Molecular & Cell Biology 6, 2005, Cosgrove, D. J.

“Growth of the plant cell wall,” pages 850-861, with permission from Nature.

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1.1. Classification of β-galactosidases and their catalytic mechanism

Carbohydrates play many important biological and physiological roles in living

organisms, and they occur in different forms such as glycoproteins, glycolipids, glycosides, and

oligo- and polysaccharides. Selective hydrolysis of glycosidic bonds specifies the breakdown of

these glyco-conjugates and their function in subsequent processes such as activation of

secondary metobolites, cell wall expansion and degradation, and turnover of signaling molecules.

Living organisms have evolved a great diversity of glycosyl hydrolases (GH) with different

substrate preferences and kinetic parameters, operating under different optimal conditions

(Davies and Henrissat, 1995; Henrissat, 1995). GH enzymes are found in all animals, plants and

microorganisms and have been classified into 113 families on the basis of amino acid sequence

similarity (CAZY database, http://www.cazy.org; (H. J. Gilbert, 1999; Henrissat, 1991a, b;

Henrissat and Bairoch, 1993, 1996; Henrissat et al., 2001; Henrissat and Davies, 1997).

GHs play many important roles in biological processes such as chemical defense,

regulation, growth and development. Among the members of the different GH families, there is

a group of enzymes with β-galactosidase activity (enzyme classification code: EC 3.2.1.23) that

hydrolyze the β-D-galactosidic bonds of lactose, oligosaccharides, polysaccharides, and

galactosides conjugated with aglycone moieties, as well as those of artificial substrates

conjugated with chromogens such as pNPGal and X-Gal. This group is distributed among four

distinct GH families, GH-1, GH-2, GH-35 and GH-42. Phylogenetic analysis shows that these

families evolved independently via separate lineages (Javeri and Uhlenbruck, 1984). The β-

galactosidases found predominantly in microorganisms belong to GH-1, GH-2, and GH-42,

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whereas most plant β-galactosidases belong to GH-35.

Enzymatic hydrolysis of the glycosidic bond by GH enzymes takes place via general acid

catalysis that requires two critical residues, a proton donor and nucleophile base. Hydrolysis

occurs via either an inverting or a retaining mechanism. With the former, a product with

opposite stereochemistry to the substrate is generated. β-galactosidases hydrolyze glycosidic

bonds by the retaining mechanism (Figure 1), where the substrate and the product have identical

stereochemical forms. All β-galactosidases fall into GH Clan A where catalytic acid and

nucleophilic base are found at the carboxyl ends of β strands 4 and 7, respectively (Davies and

Henrissat, 1995). The catalytic nucleophiles have been identified for the β-galactosidases from,

E. coli, Xanthamonas manihotas, Bacillus circulans, and human lysosome (Blanchard et al.,

2001; Juers et al., 2003; McCarter et al., 1997). Both the acid/base and nucleophile have been

putatively identified in the crystal structure of the Penicillium sp. β-galactosidase (Rojas et al.,

2004).

In addition to their critical biological functions in cells, β-galactosidases also have

numerous important practical applications, including serving as a reporter gene in genetic

engineering and for the hydrolysis of lactose or production of probiotics in the dairy and food

industries (Chantarangsee etal. 2007). Because of their importance in medicine, agriculture, and

the biotechnology industry, interest in β-galactosidases is steadily growing. For instance, in the

food industry, β-galactosidases are widely used for prolonging the post harvest shelf life of fruits

(Smith and Gross, 2000).

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Fig. 1 Retaining mechanism of enzymatic hydrolysis of the glycosidic bond. Glycosidic

oxygen is protonated by the acid catalyst (AH) and nucleophile assistance to algycone departure

is provided by the base B-. The resulting glycosyl-enzyme (enzyme substrate complex) is

hydrolyzed by a water molecule and the second nucleophile substitution at the anomeric carbon

generates a product with the same stereochemistry as the substrate. Reprinted with permission

from Davies and Henrissat (1995).

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1.2. β-Galactosidases in different organisms

Microbial β-galactosidase. Among microbial β-galactosidases, the most extensively studied

enzymes are the bacterial β-galactosidases. Of these, E. coli β-galactosidase LacZ has the long

and distinguished history in molecular biology and biotechnology. LacZ is a member of GH-2

and does not share high sequence similarity with other β-galactosidases. Detailed studies of the

catalytic mechanism and three-dimensional structure of LacZ have been carried out (Juers et al.,

2003). E. coli LacZ is a retaining β-glycosidase with a native homotetrameric structure. This

enzyme allows the bacterum to hydrolyze lactose for use as an energy and carbon source. It is

believed that other bacterial and fungal β-galactosidases may have roles similar to LacZ (Brady

et al., 1995; Rojas et al., 2004). Besides its natural substrate, LacZ can hydrolyze artificial

chromogenic substrates such as X-Gal and oNPGal (Juers et al., 2003). Another well-

characterized bacterial enzyme is the thermostable enzyme GH-42 β-galactosidase (A4-β-Gal)

from Thermus thermophilus A4 which is an extreme thermophilic organism (Hidaka et al.,

2002). This enzyme is an attractive tool for biotechnology as it can be used for lactose

processing in dairy products. It is desirable to have enzymes with greater stability that will

improve the overall efficiency and reduce the cost of such process.

Mammalian β-galactosidase. Mammalian β-galactosidases have also been extensively studied,

largely because many diseases are caused by deficiencies in these enzymes in humans and other

mammals. There are two types of mammalian β-galactosidases, including GH-35 lysosomal β-

galactosidase and GH-1 lactase phlorizin hydrolase (LPH). Lysosomal β-galactosidases are

involved in degradation of glycolipids, glycoproteins, and proteoglycans, while intestinal lactase

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in complex with phlorizin hydrolase (LPH) plays a role in digestion of milk lactose.

Diseases such as gangliosidosis and Morquio syndrome B, which lead to the

accumulation of GM1 ganglioside and its derivative asialo-GM1 ganglioside (GA1), glycoprotein-

derived oligosaccharides, and keratan sulfate, are caused by inherited deficiencies of lysosomal

β-galactosidases (Callahan, 1999; Hoogeveen et al., 1983). Because of the high frequency of

these genetic disorders in the human population, lysosomal β-galactosidases have been

extensively studied (Callahan, 1999; Galjaard et al., 1987; Gerich, 1969; Hoogeveen et al., 1983;

Nishigaki and Okada, 1998; Okada and O'Brien, 1968; Suzuki, 2006). Lysosomal β-

galactosidase is synthesized as a precursor glycoprotein. After proteolytic cleavage, a mature

protein is localized to the lysosome by endocytosis. The mature protein has the same acidic pH

optimum and kinetic parameters as the precursor. Some mutations are responsible for defective

enzymes with incorrect folding and/or susceptibilities for proteolytic degradation of mature

proteins. Other mutations affect the actual active site residues, leading to inactive lysosomal β-

galactosidases (Hoogeveen et al., 1983). Besides their natural substrates such as GM1

ganglioside, lactosyl ceramide, sialyloligosaccharide, glycopeptides, and mucopolysaccharides,

these enzymes exhibit a wide range of activities toward artificial substrates such as oNPGal and

X-Gal (DiCioccio et al., 1984; Fraser et al., 1983; Hiraiwa et al., 1986; Hiraiwa and Uda, 1986;

Javeri and Uhlenbruck, 1984; Mutoh et al., 1988).

Small intestinal lactase-phlorizin hydrolase (LPH) hydrolyzes lactose, glycosyl-L-

ceramides, and a number of aryl-L-glycosides including phlorizin (Zecca et al., 1998). In

addition to hydrolyzing lactose, phlorizin, and a number of synthetic substrates, human and pig

β-galactosidases have been found to have considerable activity with cellotriose and cellotetraose,

and a low but significant activity against cellulose (Skovbjerg et al., 1982). Dietary flavonoid

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and isoflavone glycosides are hydrolysed by lactase phlorizin hydrolase (Day et al., 2000). It is

possible that this enzyme participates in the metabolism of these plant products when consumed

by animals.

Plant β-galactosidases. β-galactosidases are found throughout the plant kingdom and play key

roles in fruit ripening, flower senescence, mobilization of carbohydrate reserves, and galactolipid

turnover, among other functions (Pressey, 1983, Bhalla and Dalling, 1984, Carey et al, 1995,

Smith et al, 1998). β-galactosidases have been purified and characterized from a variety of plant

sources, including ripening fruits and germinating seedlings of kiwifruit, tomato, apple,

strawberry and mung bean (Ross et al. 1994, Li et al., 2001, Esteban et al., 2003, Trainotti et al.,

2001). These studies also showed that β-galactosidases release stored energy for growth, release

free galactose during normal metabolic recycling of galactolipids, glycoproteins, and the cell

wall, and degrade the cell wall during senescence (Bhalla et al., 1984, Buckeridge et al 1994,

Hall et al., 1998, Smith et al., 2000,). β-galactosidases also participiate in cell wall modification

during elongation and differentiation of plant cells (Iglesias et al., 2006; Martin et al., 2008).

GH family 35 includes plant β-galactosidases that are thought to be involved in cell wall

biogenesis and modification (Jamet et al., 2006, Vervelen and Vissenberg, 2007). In plants, β-

galactosidase isozymes are encoded by multigene families. Smith et al. (2000) showed that a

family of at least seven β-galactosidases is expressed during tomato fruit development. Most of

these enzymes are large glycoproteins ranging in size from 89 to 97 kDa, with the exception of

TBG4 whose molecular mass is ~77 kDa. All seven of the β-galactosidases purified from tomato

were able to hydrolyze the synthetic substrate, pNPGal. Recombinant TBG4 expressed in yeast

was also able to hydrolyze pNPGal, lactose, although poorly, as well as chelator soluble pectin,

alkali soluble pectin, hemicellulose fractions from tomato cell walls, and commercially-prepared

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galactan to different extents (Smith et al. 2000). This suggests that galactan side chains in the

tomato cell wall are natural substrates of this enzyme and that TBG4 participates in controlling

the dynamics of these polysaccharides.

Li et al (2001) purified five isoforms of β-galactosidases from mung bean and showed

that these isoforms differ in physical-chemical and enzymatic characteristics. The mung bean

enzymes did not hydrolyze lactose; instead they were partially inhibited by lactose and D-

galactose. This indicates that these enzymes are highly specific for plant-derived substrates. An

extensive study has been carried out on strawberry β-galactosidases. Trainotti et al. (2001)

isolated three cDNAs from strawberry fruits encoding putative β-galactosidases. The enzymes

were expressed as recombinant proteins in Pichia pastoris and shown to exhibit β-galactosidase

activity with pNPGal and oNPGal. This study also showed that β-galactosidase activities in

strawberry fruits are not correlated with the expression patterns of these enzymes at the

transcriptional level (Trainotti et al., 2001). It had also been found that the enzyme activity of

tomato isoform II (TBG4) increased while mRNA level declined in fruit (Carey et al., 1995;

Smith and Gross, 2000). A possible explanation is that at given developmental stages the enzyme

becomes more stable but the expression level decreases.

Amino acid sequence analysis of plant β-galactosidases revealed that these enzymes are

divided into two subgroups with different polypeptide sizes. The larger β-galactosidases possess

well-conserved C-terminal lectin-like SEUL (sea urchin egg lectin) type domains that are

approximately 100 amino acids long and contain seven highly conserved cysteine residues. A

100-amino acid long C-terminal domain is common to many β-galactosidases. These lectin-like

domains show homology to animal lectin proteins rather than plant lectins. SEUL lectins from

sea urchin eggs have been shown to be L-rhamnose- and D-galactose-specific homodimers

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(Ozeki et al., 1995). Although there are no experimental data on the specific function of this

domain in plants (Tanthanuch et al., 2008), it has been proposed that the lectin-like domain could

enhance the affinity of the enzymes for their substrates, thereby increasing catalytic efficiency

(Ahn et al., 2007) and possibly also enzyme stability (Trainotti et al. 2001).

1.3. Arabidopsis thaliana β-galactosidase

The National Scientific Foundation launched the Arabidopsis 2010 Program in 2001, one

of the objectives was to determine the functions of all predicted genes in the recently-completed

genome sequence. As a part of this effort, the “Functional genomics of the Arabidopsis β-

galactosidase gene family” project aimed to determine the functions of all β-galactosidase genes

in this model plant.

A family of 17 putative β-galactosidase genes has been identified in Arabidopsis

thaliana, clustering into seven subfamilies based on sequence similarity (Figure 2) (Ahn et al.,

2007; Gantulga et al., 2008). All of the Arabidopsis β-galactosidase proteins except Gal-14 have

an N-terminal signal peptide, which may target the enzymes through the vesicular pathway to the

outside of the cell, consistent with a proposed role in cell wall modification. Most of these

enzymes appear to be glycoproteins with 1-9 potential N-glycosylation sites. Ten of the β-

galactosidases have the C-terminal SUEL type lectin-like domain described above.

Among the four subfamilies that comprise the β-galactosidase genes in Arabidopsis

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A)

B)

BGAL2 -----MSMHFRNKAWIILAILCFSSLIHSTEAVVTYDHKALIINGQRRILISGSIHYPRS 55 BGAL12 -----MGLNFREKAWILLGILCCSSLICSVKAIVTYDRKAVIINGQRRILLSGSIHYPRS 55 BGAL4 -----MVLNFRDKSCIFLAILCCLSLSCIVKASVSYDRKAVIINGQRRILLSGSIHYPRS 55 BGAL1 MGSKPNAMKNVVAMAAVSALFLLGFLVCSVSGSVSYDSRAITINGKRRILISGSIHYPRS 60 BGAL3 ---MGTGDSASRLILWFCLGFLILGVGFVQCG-VTYDRKALLINGQRRILFSGSIHYPRS 56 BGAL5 ---MGTTILVLSKILTFLLTTMLIGSSVIQCSSVTYDKKAIVINGHRRILLSGSIHYPRS 57 . . *:** :*: ***:****:********* BGAL2 TPEMWPDLIKKAKEGGLDVIQTYVFWNGHEPSPGNYYFQDRYDLVKFTKLVHQAGLYLDL 115 BGAL12 TPEMWPDLIQKAKDGGLDVIQTYVFWNGHEPSPGQYYFEDRYDLVKFIKVVQQAGLYVHL 115 BGAL4 TPEMWPGLIQKAKEGGLDVIETYVFWNGHEPSPGQYYFGDRYDLVKFIKLVHQAGLYVNL 115 BGAL1 TPEMWPDLIRKAKEGGLDVIQTYVFWNGHEPSPGKYYFEGNYDLVKFVKLVQQSGLYLHL 120 BGAL3 TPDMWEDLIQKAKDGGIDVIETYVFWNLHEPSPGKYDFEGRNDLVRFVKTIHKAGLYAHL 116 BGAL5 TPEMWEDLIKKAKDGGLDVIDTYVFWNGHEPSPGTYNFEGRYDLVRFIKTIQEVGLYVHL 117 **:** .**:***:**:***:****** ****** * * .. ***:* * ::: *** .* BGAL2 RIGPYVCAEWNFGGFPVWLKYVPGMVFRTDNEPFKIAMQKFTKKIVDMMKEEKLFETQGG 175 BGAL12 RIGPYVCAEWNFGGFPVWLKYVPGMVFRTDNEPFKAAMQKFTEKIVRMMKEEKLFETQGG 175 BGAL4 RIGPYVCAEWNFGGFPVWLKFVPGMAFRTDNEPFKAAMKKFTEKIVWMMKAEKLFQTQGG 175 BGAL1 RIGPYVCAEWNFGGFPVWLKYIPGISFRTDNGPFKAQMQRFTTKIVNMMKAERLFESQGG 180 BGAL3 RIGPYVCAEWNFGGFPVWLKYVPGISFRTDNEPFKRAMKGFTERIVELMKSENLFESQGG 176 BGAL5 RIGPYVCAEWNFGGFPVWLKYVDGISFRTDNGPFKSAMQGFTEKIVQMMKEHRFFASQGG 177 ********************:: *: ***** *** *: ** :** :** ..:* :***

11

BGAL2 PIILSQIENEYGPMQWEMGAAGKAYSKWTAEMALGLSTGVPWIMSKQEDAPYPIIDTCNG 235 BGAL12 PIILSQIENEYGPIEWEIGAPGKAYTKWVAEMAQGLSTGVPWIMCKQDDAPNSIINTCNG 235 BGAL4 PIILAQIENEYGPVEWEIGAPGKAYTKWVAQMALGLSTGVPWIMCKQEDAPGPIIDTCNG 235 BGAL1 PIILSQIENEYGPMEYELGAPGRSYTNWAAKMAVGLGTGVPWVMCKQDDAPDPIINACNG 240 BGAL3 PIILSQIENEYGRQGQLLGAEGHNYMTWAAKMAIATETGVPWVMCKEDDAPDPVINTCNG 236 BGAL5 PIILSQIENEFEPDLKGLGPAGHSYVNWAAKMAVGLNTGVPWVMCKEDDAPDPIINTCNG 237 ****:*****: :*. *: * .*.*:** . *****:*.*::*** .:*::*** BGAL2 FYCEGFKPNSDNKPKLWTENWTGWFTEFGGAIPNRPVEDIAFSVARFIQNGGSFMNYYMY 295 BGAL12 FYCENFKPNSDNKPKMWTENWTGWFTEFGGAVPYRPAEDIALSVARFIQNGGSFINYYMY 295 BGAL4 YYCEDFKPNSINKPKMWTENWTGWYTDFGGAVPYRPVEDIAYSVARFIQKGGSLVNYYMY 295 BGAL1 FYCDYFSPNKAYKPKMWTEAWTGWFTKFGGPVPYRPAEDMAFSVARFIQKGGSFINYYMY 300 BGAL3 FYCDSFAPNKPYKPLIWTEAWSGWFTEFGGPMHHRPVQDLAFGVARFIQKGGSFVNYYMY 296 BGAL5 FYCDYFTPNKPYKPTMWTEAWSGWFTEFGGTVPKRPVEDLAFGVARFIQKGGSYINYYMY 297 :**: * **. ** :*** *:**:*.***.: **.:*:* .******:*** :***** BGAL2 YGGTNFDRTAG-VFIATSYDYDAPIDEYGLLREPKYSHLKELHKVIKLCEPALVSVDPTI 354 BGAL12 HGGTNFDRTAG-EFIATSYDYDAPLDEYGLPREPKYSHLKRLHKVIKLCEPALVSADPTV 354 BGAL4 HGGTNFDRTAG-EFMASSYDYDAPLDEYGLPREPKYSHLKALHKAIKLSEPALLSADATV 354 BGAL1 HGGTNFGRTAGGPFIATSYDYDAPLDEYGLERQPKWGHLKDLHRAIKLCEPALVSGEPTR 360 BGAL3 HGGTNFGRTAGGPFVTTSYDYDAPIDEYGLIRQPKYGHLKELHRAIKMCEKALVSADPVV 356 BGAL5 HGGTNFGRTAGGPFITTSYDYDAPIDEYGLVQEPKYSHLKQLHQAIKQCEAALVSSDPHV 357 :*****.**** *:::*******:***** ::**:.*** **:.** .* **:* :. . BGAL2 TSLGDKQEIHVFKSKT-SCAAFLSNYDTSSAARVMFRGFPYDLPPWSVSILPDCKTEYYN 413 BGAL12 TSLGDKQEAHVFKSKS-SCAAFLSNYNTSSAARVLFGGSTYDLPPWSVSILPDCKTEYYN 413 BGAL4 TSLGAKQEAYVFWSKS-SCAAFLSNKDENSAARVLFRGFPYDLPPWSVSILPDCKTEVYN 413 BGAL1 MPLGNYQEAHVYKSKSGACSAFLANYNPKSYAKVSFGNNHYNLPPWSISILPDCKNTVYN 420 BGAL3 TSIGNKQQAHVYSAESGDCSAFLANYDTESAARVLFNNVHYNLPPWSISILPDCRNAVFN 416 BGAL5 TKLGNYEEAHVFTAGKGSCVAFLTNYHMNAPAKVVFNNRHYTLPAWSISILPDCRNVVFN 417 :* :: :*: : . * ***:* . .: *:* * . * **.**:******:. :* BGAL2 TAKIRAPT--ILMKMIPTSTK-FSWESYNEGSPSSNEAGTFVKDGLVEQISMTRDKTDYF 470 BGAL12 TAKVQVRTSSIHMKMVPTNTP-FSWGSYNEEIPSANDNGTFSQDGLVEQISITRDKTDYF 472 BGAL4 TAKVNAPS--VHRNMVPTGTK-FSWGSFNEATPTANEAGTFARNGLVEQISMTWDKSDYF 470 BGAL1 TARVGAQTSRMKMVRVPVHGG-LSWQAYNED-PSTYIDESFTMVGLVEQINTTRDTSDYL 478 BGAL3 TAKVGVQTS--QMEMLPTDTKNFQWESYLEDLSSLDDSSTFTTHGLLEQINVTRDTSDYL 474 BGAL5 TATVAAKTS--HVQMVPSGSILYSVARYDEDIATYGNRGTITARGLLEQVNVTRDTTDYL 475 ** : . : :* . : * .: :: **:**:. * *.:**: BGAL2 WYFTDITIGSDESFLKTGDNPLLTIFSAGHALHVFVNGLLAGTSYGALSNSKLTFSQNIK 530 BGAL12 WYLTDITISPDEKFL-TGEDPLLTIGSAGHALHVFVNGQLAGTAYGSLEKPKLTFSQKIK 531 BGAL4 WYITDITIGSGETFLKTGDSPLLTVMSAGHALHVFVNGQLSGTAYGGLDHPKLTFSQKIK 530 BGAL1 WYMTDVKVDANEGFLRNGDLPTLTVLSAGHAMHVFINGQLSGSAYGSLDSPKLTFRKGVN 538 BGAL3 WYMTSVDIGDSESFLHGGELPTLIIQSTGHAVHIFVNGQLSGSAFGTRQNRRFTYQGKIN 534 BGAL5 WYTTSVDIKASESFLRGGKWPTLTVDSAGHAVHVFVNGHFYGSAFGTRENRKFSFSSQVN 535 ** *.: : .* ** *. * * : *:***:*:*:** : *:::* . :::: :: BGAL2 LSVGINKLALLSTAVGLPNAGVHYETWNTGILGPVTLKGVNSGTWDMSKWKWSYK-IGLR 589 BGAL12 LHAGVNKLALLSTAAGLPNVGVHYETWNTGVLGPVTLNGVNSGTWDMTKWKWSYKQIGTK 591 BGAL4 LHAGVNKIALLSVAVGLPNVGTHFEQWNKGVLGPVTLKGVNSGTWDMSKWKWSYK-IGVK 589 BGAL1 LRAGFNKIAILSIAVGLPNVGPHFETWNAGVLGPVSLNGLNGGRRDLSWQKWTYK-VGLK 597 BGAL3 LHSGTNRIALLSVAVGLPNVGGHFESWNTGILGPVALHGLSQGKMDLSWQKWTYQ-VGLK 593 BGAL5 LRGGANKIALLSVAVGLPNVGPHFETWATGIVGSVVLHGLDEGNKDLSWQKWTYQ-AGLR 594 * * *::*:** *.****.* *:* * *::*.* *:*:. * *:: **:*: * : BGAL2 GEAMSLHTLAGSSAVKWWIKGFVVKK-QPLTWYKSSFDTPRGNEPLALDMNTMGKGQVWV 648 BGAL12 GEALSVHTLAGSSTVEWKEGSLVAKK-QPLTWYKSTFDSPTGNEPLALDMNTMGKGQMWI 650 BGAL4 GEALSLHTNTESSGVRWTQGSFVAKK-QPLTWYKSTFATPAGNEPLALDMNTMGKGQVWI 648 BGAL1 GESLSLHSLSGSSSVEWAEGAFVAQK-QPLTWYKTTFSAPAGDSPLAVDMGSMGKGQIWI 656 BGAL3 GEAMNLAFPTNTPSIGWMDASLTVQKPQPLTWHKTYFDAPEGNEPLALDMEGMGKGQIWV 653 BGAL5 GESMNLVSPTEDSSVDWIKGSLAKQNKQPLTWYKAYFDAPRGNEPLALDLKSMGKGQAWI 654 **::.: : . : * .:. :: *****:*: * :* *:.***:*: ***** *: BGAL2 NGHNIGRHWPAYTARGNCGRCNYAGIYNEKKCLSHCGEPSQRWYHVPRSWLKPFGNLLVI 708 BGAL12 NGQNIGRHWPAYTARGKCERCSYAGTFTEKKCLSNCGEASQRWYHVPRSWLKPTNNLVIV 710 BGAL4 NGRNIGRHWPAYKAQGSCGRCNYAGTFDAKKCLSNCGEASQRWYHVPRSWLKSQ-NLIVV 707 BGAL1 NGQSLGRHWPAYKAVGSCSECSYTGTFREDKCLRNCGEASQRWYHVPRSWLKPSGNLLVV 716 BGAL3 NGESIGRYWTAF-ATGDCSHCSYTGTYKPNKCQTGCGQPTQRWYHVPRAWLKPSQNLLVI 712 BGAL5 NGQSIGRYWMAF-AKGDCGSCNYAGTYRQNKCQSGCGEPTQRWYHVPRSWLKPKGNLLVL 713 **..:**:* *: * *.* *.*:* : .** **:.:********:***. **:::

12

BGAL2 FEEWGGDPSGISLVKRTAK----------------------------------------- 727 BGAL12 LEEWGGEPNGISLVKRTAK----------------------------------------- 729 BGAL4 FEELGGDPNGISLVKRT------------------------------------------- 724 BGAL1 FEEWGGDPNGITLVRREVDSVCADIYEWQSTLVNYQLHASGKVNKPLHPKAHLQCGPGQK 776 BGAL3 FEELGGNPSTVSLVKRSVSGVCAEVSEYHPNIKNWQIESYGKGQTFHRPKVHLKCSPGQA 772 BGAL5 FEELGGDISKVSVVKRSVN----------------------------------------- 732 :** **: . :::*:* BGAL2 ------------------------------------------------------------ BGAL12 ------------------------------------------------------------ BGAL4 ------------------------------------------------------------ BGAL1 ITTVKFASFGTPEGTCGSYRQGSCHAHHSYDAFNKLCVGQNWCSVTVAPEMFGGDPCPNV 836 BGAL3 IASIKFASFGTPLGTCGSYQQGECHAATSYAILERKCVGKARCAVTISNSNFGKDPCPNV 832 BGAL5 ------------------------------------------------------------ BGAL2 --------------------- BGAL12 --------------------- BGAL4 --------------------- BGAL1 MKKLAVEAVCA---------- 847 BGAL3 LKRLTVEAVCAPETSVSTWRP 853 BGAL5 ---------------------

Fig. 2 Phylogenetic analysis of Arabidopsis thaliana β-galactosidases. A) Phylogenetic tree of

the β−galactosidase family derived from an alignment of the amino acid sequences of 17

members. The family is divided into seven subfamilies, of which Subfamily III, which includes

Gal-1 through Gal-5 and Gal-12, is the largest. B) Alignment of Subfamily III enzymes that

share 60-81% sequence identity with each other. Red color indicates putative active site residues

and yellow color indicates conserved amino acid residues.

13

thaliana , subfamily III, which includes Gal-1, Gal-2, Gal-3, Gal-4, Gal-5 and Gal-12, is the

largest. Most of the research on β-galactosidases has been focused on this subfamily because the

genes are believed to be the most redundant and because the homologous genes have been

studied for other organisms, which are believed to be involved in cell wall modification.

Plant cell walls are composed of independent but interacting networks of carbohydrates

along with proteins, and aromatic substances (Carpita and Gibeaut, 1993; McCann and Roberts,

1991) (Fig. 3). Interacting with this complex matrix are several hundred enzymes and other

proteins that carry out many functions, from wall assembly and disassembly to defense against

pathogens. In recent years, several cell-wall structural proteins and enzymes, and their

respective genes, have been identified (Henrissat et al., 2001; Richmond and Somerville, 2001).

According to a recent microarray study, β-galactosidase genes are among these genes specific to

cell wall dynamics (Imoto et al., 2005).

Analysis of several Arabidopsis β-galactosidase genes using promoter-GUS fusions

uncovered differences in temporal and spatial expression among subfamily members (Perez,

2004). This study also showed that the galactosyl content of the cell wall is increased and total

β-galactosidase activity is decreased in a Gal-1 knockout mutant relative to wild-type plants.

Iglesias et al. (2006) reported β-galactosidase activity on xyloglucan oligosaccharides in the

apoplastic fluid in Arabidopsis. Proteomics analysis of apoplastic fluid from intact Arabidopsis

rosettes identified numerous GH enzymes, among them Gal-1 (Boudart et al., 2005). All these

studies indicate that β-galactosidases function in cell wall dynamics by helping to reorganize

polysaccharide molecules during development. β-galactosidases also appear to function in

degrading the plant cell wall during senescence (O'Donoghue et al., 2002). Polysaccharide

14

Fig. 3 A model of plant cell wall structure. The major components of plant cell walls include

cellulose, hemicellulose, cross-linking glycans, pectin and proteins. Cellulose microfibrils are

cross-linked through hemicellulose and pectin. Hemicellulosic xyloglucan and pectic

rhamnogalacturonan contain side chains that are rich in galactan. Galactan is a linear polymer of

galactose that is susceptible to hydrolysis by β-galactosidases. Reprinted with permission from

Cosgrove (2005).

15

metabolism within the extracellular matrix allows flexibility in cell wall structure even in mature

cell walls.

Although past experiments have advanced our understanding of β-galactosidases and

suggested involvement of the subfamily III genes in cell wall dynamics, key questions remain

unanswered, including the natural substrate specificities and subcellular localization of the gene

products. Isolation and purification of native enzymes and recombinant proteins would greatly

facilitate answering these questions. It is also desirable to study not a single gene but the entire

subfamily to show whether the sequence similarities among the genes translate into similarities

in function, and whether there is specialization among the enzymes for certain organs and

tissues. The goal of the work described in this dissertation is to address these questions by

systematic characterization of the Arabidopsis Subfamily III β-galactosidases.

16

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22

Chapter 2

The Arabidopsis At1g45130 and At3g52840 genes encode β-galactosidases with activity

toward cell wall polysaccharides.

This work was published previously and reprinted with kind permission from Elsevier:

Phytochemistry, “The Arabidopsis At1g45130 and At3g52840 genes encode β-galactosidases

with activity toward cell wall polysaccharides.” Vol. 69, 2008, 1661-1670, D. Gantulga, Y.

Turan, D. R. Bevan, and A. Esen.

Keywords: Arabidopsis; Cruciferae; Cell wall; Glycosyl hydrolase family 35; β-Galactosidase;

β-Galactosides; Galactan

Abbreviations: Arabidopsis thaliana, Arabidopsis; Gal-5, (AtBGal5)–β−galactosidase-5; Gal-2,

(AtBGal2)–β-galactosidase-2; pNP, para-nitrophenyl; GH, glycosyl hydrolase.

23

Abstract

The Arabidopsis genes At1g45130 and At3g52840 encode the β-galactosidase isozymes Gal-5

and Gal-2 that belong to Glycosyl Hydrolase Family 35 (GH 35). The two enzymes share 60%

sequence identity with each other and 38-81% with other plant β-galactosidases that are reported

to be involved in cell wall modification. We studied organ-specific expression of the two

isozymes. According to our western blot analysis using peptide-specific antibodies, Gal-5 and

Gal-2 are most highly expressed in stem and rosette leaves. We show by dot-immunoblotting

that Gal-5 and Gal-2 are associated with the cell wall in Arabidopsis. We also report expression

of the recombinant enzymes in P. pastoris and describe their substrate specificities. Both

enzymes hydrolyze the synthetic substrate para-nitrophenyl-β-D-galactopyranoside and display

optimal enzyme activity between pH 4.0 and 4.5, similar to the pH optimum reported for other

well-characterized plant β-galactosidases. Both Gal-5 and Gal-2 show a broad specificity for the

aglycone moiety and a strict specificity for the glycone moiety in that they prefer galactose and

its 6-deoxy analogue, fucose. Both enzymes cleave β-(1, 4) and β-(1, 3) linkages in galacto-

oligosaccharides and hydrolyze the pectic fraction of Arabidopsis cell wall. These findings

suggest that Gal-5 and Gal-2 could be involved in the modification of cell wall polysaccharides.

24

1. Introduction

Carbohydrates play many important biological and physiological roles in living

organisms, and they occur in different forms such as glycoproteins, glycolipids, glycosides, and

oligo- and polysaccharides. Selective hydrolysis of glycosidic bonds specifies the breakdown of

these glycoconjugates and their function in subsequent biochemical processes. Living organisms

have evolved a great diversity of glycosyl hydrolases (GH) with different substrate specificity

and kinetic parameters, operating under different optimal conditions (Henrissat, 1995). Glycosyl

hydrolases are found in all animals, plants and microorganisms, and they are classified into 111

families on the basis of amino acid sequence similarity (CAZY database, http://www.cazy.org;

(H. J. Gilbert, 1999). The GH family 35 includes plant β-galactosidases (EC 3.2.1.23) that are

thought to be involved in cell wall biogenesis and modification (Jamet et al., 2006, Vervelen and

Vissenberg, 2007). In plants, β-galactosidase isozymes are encoded by a multigene family. The

presence of β-galactosidases as a moderately large enzyme family is indicative of the structural

complexity and heterogeneity of cell wall polysaccharides. Depending on developmental stages

and cell differentiation, the different isozymes may act on the same or different cell wall

polysaccharides (Smith and Gross, 2000; Perez, 2004; Kotake et al., 2005; Ishimaru et al., 2005;

Jamet et al., 2006; Chantarangsee et al., 2007).

A growing body of evidence suggests that GH Family 35 β-galactosidases participate in

loosening of the cell wall during growth and degrade cell wall components during ripening of

fruits and senescence. Recently, many β-galactosidases have been identified and purified from

different plant sources. For example, Smith and Gross (2000) showed that a family of at least

seven β−galactosidases is expressed during tomato fruit development. Furthermore, these authors

25

expressed the tomato β−galactosidase isozyme 4 (TBG4) in yeast and studied its natural

substrate specificity. TBG4 hydrolyzed chelator-soluble pectin, alkali-soluble pectin,

hemicellulose from tomato cell walls, and commercially prepared galactan. Li et al. (2001)

purified five isoforms of β−galactosidases from mung bean and showed that they differ with

respect to enzymatic characteristics and substrate specificities. Kotake et al. (2004) isolated a

β−galactosidase from radish (RsbGal), which specifically hydrolyzed β-(1, 6) and β-(1, 3)

linkages in arabinogalactan protein. β-Galactosidase I purified from ripe carambola (A.

carambola) was active in hydrolyzing β-(1, 4)-linked spruce and β-(1, 6)/ β-(1, 3)-linked gum

arabic galactan, and alkali-soluble hemicelluloses of carambola cell wall (Balasubramaniam et

al, 2005). Furthermore, papaya β-galactosidase isoforms differentially hydrolyzed cell wall

during fruit ripening (Lazan et al., 2004).

Despite the status of Arabidopsis thaliana as a model plant with a completely sequenced

genome, not much is known about its β-galactosidases. There are 17 putative GH family 35 β-

galactosidases (17 BGALs) in Arabidopsis (Ahn et al., 2007). According to a recent microarray

study, the β-galactosidase genes are among 765 genes specific to cell wall dynamics (Imoto et

al., 2005). Perez (2004) studied expression of β-galactosidases by promoter-GUS fusion and

found that temporal and spatial expression of β-galactosidase genes are differentially regulated.

She also reported that the galactosyl content of the cell wall increases and total β-galactosidase

activity decreases in Gal-1 knockout mutant. Iglesias et al. (2006) reported β-galactosidase

activity on xyloglucan oligosaccharides in the apoplastic fluid. To corroborate further the role of

β-galactosidases in cell wall modification, it is essential to determine their substrate specificity,

by isolating and characterizing the proteins. To this end, Ahn et al. (2007) expressed Gal-4

(At5g56870), one of the seventeen β-galactosidases, in E. coli and insect cells. It was shown that

26

in addition to synthetic substrates, Gal-4 hydrolyzes β-(1, 3) and β-(1, 4) linked galacto-

oligosaccharides (Ahn et al., 2007).

It is likely that the 17 BGALs differ in temporal and spatial expression and in their ability

to hydrolyze various cell wall polysaccharides. For annotating them and understanding their

function better, it is desirable to determine the expression profile and substrate specificity of all

17 BGALs. Currently, we are focused on the largest subfamily (Gal-1 through Gal-5 and Gal-

12), which consists of six genes, amongst the 17 BGALs. Of these, we studied Gal-5 and Gal-2

for this paper. First, we conducted database analyses using microarray data available for these

genes. Second, we studied spatial expression and subcellular localization by immunoblotting

using peptide-specific antibodies. Third, we purified the recombinant proteins expressed in P.

pastoris and investigated their substrate specificity using synthetic galactosides and natural

polysaccharides derived from Arabidopsis cell wall.

2. Results and Discussion

2.1. In silico analysis of Gal-5 and Gal-2

BLAST searches of the NCBI database using Gal-5 and Gal-2 sequences as queries show

that the sequences of 20 other plant β-galactosidases share 38-81% identity with all Arabidopsis

β-galactosidases except Gal-17. Gal-5 and Gal-2 proteins share 60% sequence identity with each

other. The sequence differences among β-galactosidases suggest differences in their enzymatic

properties (e.g., substrate specificity) and biological functions.

27

In-silico characterization of predicted protein sequences of Gal-5 and Gal-2 using SignalP and

TargetP shows that these proteins contain a signal peptide that targets them to the endoplasmic

reticulum from which they can be secreted to the cell wall via vesicular pathways (Bendtsen et

al., 2004; Emanuelsson et al., 2000). Phylogenetic analysis of Gal-5 and Gal-2 proteins shows

that they cluster with other plant β-galactosidases that are known to be cell wall modifying

enzymes (Fig. 1). In tomato fruit, the galactosyl content of cell wall and fruit firmness depend on

two β-galactosidases, TBG6 and TBG4 (Smith et al., 2002), which are closely related to Gal-5

and Gal-2, respectively (Fig. 1).

2.2. Microarray data for expression of At1g45130 (Gal-5) and At3g52840 (Gal-2)

transcripts

To study the expression profiles of the Gal-5 and Gal-2 genes, we used microarray data

from the integrated database Genevestigator (Zimmermann et al., 2004). We analyzed the

relative abundance of At1g45130 (Gal-5) and At3g52840 (Gal-2) transcripts for different tissues,

developmental stages, and various stress conditions. Table 1 shows mean gene expression data

across many microarray experiments carried out in different laboratories. Relatively small

standard errors for the mean expression values indicate the consistency and reliability of the data.

The mean expression of Gal-5 is high in stem, root and silique, while the mean expression of

Gal-2 is high in the petiole of rosette leaves and stem. The transcript levels do not change

notably throughout developmental stages from seedling to adult plants. Gal-5 and Gal-2

transcripts were absent in the pollen (data not shown). It is possible that other members of BGAL

28

Fig. 1. Phylogenetic tree of selected plant β-galactosidases. The tree was constructed from

the alignment of the amino acid sequences using Clustal W software in

http://align.genome.jp/. AtGal-1, A. thaliana β-galactosidase-1/AtBGAL1 (NP187988); AtGal-

2, A. thaliana β-galactosidase-2/AtBGAL2 (NP190852); AtGal-3, A. thaliana β-galactosidase-

3/AtBGAL3 (NP849506); AtGal-4, A. thaliana β-galactosidase-4/AtBGAL4 (CAB64740);

AtGal-5, A. thaliana β-galactosidase-5/AtBGAL5 (NP175127); AtGal-12, A. thaliana β-

galactosidase-12/AtBGAL12 (NP849553); Pear BGal, P. pyrifolia (BAB21492); Papaya BGal,

C. papaya (AAC77377); TBG1, L. esculentum (AAF21626); TBG2, L. esculentum

(AAF70821); TBG3, L. esculentum (AAF70822); TBG4, L. esculentum (AAC25984); TBG5,

L. esculentum (AAF70824); TBG6, L. esculentum (AAF70825); TBG7, L. esculentum

(AAF70823); Mung bean BGal, V. radiata (AAF67341); Peach BGal, P. persica

(AAW47739); Lupin BGal, L. angustifolius (CAA09467); RsbGal, R. sativus β-galactosidase 1

(BAD20774); Human BGal, H. sapiens (P16278).

29

Table 1. Expression profile of Gal-5 and Gal-2 using microarray data

Organ/tissue

# of chips

At1g45130 Gal-5

At3g52840 Gal-2

At4g05320 UBQ10

Root 200 11.67±0.04 8.55±0.06 14.7±0.02 Petiole 12 10.62±0.11 12.82±0.2 14.8±0.07 Rosette 604 8.93±0.05 11.14±0.04 14.7±0.01 Stem 7 12±0.04 11.26±0.22 14.6±0.24 Cauline leaf 3 7.3±0.33 10.69±0.11 14.8±0.04 Flower 58 9.94±0.25 9.12±0.11 14.5±0.07 Silique 11 11.83±0.33 10.12±0.23 14.8±0.04 Seedling 386 10.57±0.05 10.9±0.03 14.8±0.01

Second column shows the number of microarray chips available for a given tissue. The organ/tissue specific expression levels for Gal-5 (At1g45130) and Gal-2 (At3g52840) as log2(n) are shown along with the standard errors in columns three and four. Ubiquitin10 (UBQ10/Atg05320) used as controls for comparison are shown in column five. The Gene Atlas Tool of Genevestigator (https://www.genevestigator.ethz.ch/at/) was used for analysis.

30

family are expressed in pollen to rescue Gal-5 and Gal-2. Indeed, Hruba et al. (2005) showed

that Gal-7, Gal-11 and Gal-13 are specifically expressed in the pollen. It is worth mentioning

that microarray results are in good agreement with recent RT-PCR results for selected organs

(Ahn et al., 2007).

Comprehensive microarray data are available for Arabidopsis genes expressed in shoots

and roots under various stress conditions (http://www.weigelworld.org/resources/microarray/

AtGenExpress). According to the microarray data, expression levels of Gal-5 and Gal-2

transcripts are not significantly different under most of the stress conditions, except UV-B light

and osmotic (mannitol) stresses. In comparison to the control, maximal change (~7-fold increase)

of Gal-2 expression is in roots (harvested after 3 hours) treated with UV-B light. For Gal-5,

maximal change (~3-fold decrease) is in root (harvested after 24 hours) treated by osmotic stress.

An RT-PCR study of β-galactosidase gene expression in plant organs under various stress

conditions and hormone treatments may further corroborate the role of BGALs in cell wall

modification, by demonstrating coordinated expression of β-galactosidase genes and the genes

whose products are known to be cell wall-specific (e.g. expansin, polygalacturonase).

2.3. Expression of Gal-5 and Gal-2 in Arabidopsis: western blotting studies using peptide-

specific antibodies

Before the injection of immunogens, rabbit preimmune sera were tested by ELISA for

immunoreactivity with total protein extracts of Arabidopsis and synthetic peptides (data not

shown). Two rabbits with the lowest preimmune serum reactivity were chosen for the injection

of Gal-5 and Gal-2 peptide conjugates, respectively. The peptide-specific antisera were tested for

31

their specificity by dot blotting. Anti- Gal-2 peptide antibody recognized Gal-2 synthetic peptide

and the recombinant Gal-2 protein (E. coli and P. pastoris expressed), and anti- Gal-5 peptide

antibody recognized Gal-5 peptide and recombinant Gal-5. Preimmune sera did not show any

detectable immunoreactivity with Gal-5 and Gal-2 on the blots.

The expression pattern of the two β-galactosidases in different Arabidopsis tissues was

studied by western blotting using antibodies raised against Gal-5 and Gal-2 specific peptides.

Immunoreactive bands of ~75 kD were detected in 4 to 5-week old plants (Fig. 2A), indicating

Gal-5 and Gal-2 proteins do not undergo proteolytic processing that has been reported to produce

35- to 50-kD fragments for β-galactosidases in apple (Ross et al., 1995), lupin (Buckeridge et al.,

1994) and radish (Kotake et al., 2005). Such fragments were not detected on immunoblots even

after using high protein loads (Fig. 2B). Under our experimental conditions, levels of Gal-2

protein were similar in all organs (root, leaf, stem, flower, and silique), whereas Gal-5 levels

were different. Gal-5 levels were higher in stem and leaves, but lower in roots and silique. We

compared our results with the previous studies on expression of β-galactosidase genes by Perez

(2004) and Ahn et al. (2007). These authors found Gal-2 transcripts with moderate levels of

expression in all organs, which is in agreement with our result for Gal-2. In the case of Gal-5,

they found higher level of expression in roots than we did. We propose that this difference in

Gal-5 expression is due to the difference in transcriptional and translational stages of regulation,

though we do not exclude other factors, such as plant age, growth conditions, and extraction

methods used in the experiments. It is worth mentioning that there is agreement between our data

and those of Perez (2004) in that Gal-5 is not detectable in mature roots, although Perez (2004)

found Gal-5 expression in root elongation and root hair zones of juvenile plants.

32

Fig. 2. Organ-specific expression of Gal-5 and Gal-2. A. Western blot and B. Coomassie

Blue stained SDS-PAGE. Total proteins (30 µg) from Arabidopsis (4 weeks old) root, petiole of

rosette leaves, rosette leaves, stems, cauline leaves, flowers, and siliques were separated by 10%

SDS-PAGE and transferred to nitrocellulose membranes. Identical membranes were incubated

with rabbit preimmune sera and immune antisera against Gal-5 and Gal-2 peptides. The arrow

marks the position of immunoreactive bands.

33

2.4. Cell wall localization of Gal-5 and Gal-2 proteins: dot blotting

We isolated cell wall from rosette leaves of Arabidopsis to confirm the presence of Gal-5

and Gal-2 proteins in the cell wall. Five different fractions (S1, soluble 1; S2, soluble 2; S3,

soluble 3; CW4, extractable with CaCl2; and CW5, extractable with LiCl) were obtained. These

fractions were assayed for β-galactosidase activity using pNPGal as a substrate. S2 and S3

fractions did not have detectable activity. Specific activities of S1, CW4, and CW5 were 0.06,

0.12, and 20.1 nmole pNP/min/mg, respectively, indicating that fraction CW5 (LiCl-soluble)

had the highest specific β-galactosidase activity. Fraction CW5 contained the lowest amount of

protein (Fig. 3, bottom row) among the three fractions. The immunoblotting data showed that

antiserum to intact Gal-2 protein had high immunoreactivity (Fig. 3, top blot) with fraction CW4

(CaCl2-soluble) and weak immunoreactivity with fractions S1 and CW5. However, this

antiserum is not specific for Gal-2; it recognizes also other cell wall-bound β-galactosidases. In

contrast, the reactivity of the Gal-2 peptide-specific antiserum was strongest with fraction CW5

(Fig. 3, second blot), indicating that Gal-2 is enriched in CW5 and it requires LiCl for complete

release from the cell wall. In the case of Gal-5, the preimmune serum from the rabbit immunized

with Gal-5 peptide had considerable background activity with cell wall components (Fig. 3,

fourth blot). Although the immune serum from the same rabbit reacted more strongly with

fractions CW4 and CW5 than the preimmune serum, the difference between the specific and the

background reactions was not as striking as for Gal-2. Taken together, our enzyme activity and

dot immunoblotting data indicate both Gal-5 and Gal-2 are present in, and tightly associated

with, the cell wall in Arabidopsis. ELISA data (not shown) also confirmed that Gal-5 and Gal-2

34

Fig. 3. Localization of Gal-5 and Gal-2 in cell wall fractions by dot blotting. Cell walls were

isolated from Arabidopsis rosette leaves. Three fractions (S1, soluble; CW4, CaCl2-soluble;

CW5, LiCl-soluble) with β-galactosidase activity were spotted on nitrocellulose strips at the

same place multiple times. Identical membranes with protein spots were incubated with anti-

whole Gal-2, anti- Gal-5 peptide-, and anti- Gal-2 peptide- specific antisera. Total protein spots

were stained with Coomassie Blue R-250. These dot immunoblotting data show the presence of

Gal-5 and Gal-2 in the cell wall.

35

proteins were present in cell wall fractions, supporting our hypothesis that Gal-5 and Gal-2

proteins are bound to the cell wall.

2.5. Expression of Gal-5 and Gal-2 in P. pastoris and purification

The recombinant proteins were expressed under the control of the AOX (alcohol oxidase)

promoter in P. pastoris. Gal-5, 700 amino acids long, (79-kD protein, calculated) and Gal-2, 719

amino acids long, (81-kD protein, calculated), were expressed and secreted into the culture

medium. Optimization of induction and time-course studies of expression were done to obtain

the best expression level for recombinant Gal-5 and Gal-2. Results from the induction time

course (data not shown) showed that β-galactosidase activity was secreted into the culture

medium and was detectable after 24 hrs of induction on 1% methanol, and it peaked after 72-96

hours. While β-galactosidase activity of Gal-5 and Gal-2 transformants increased during the

course of induction, no detectable activity was observed in the control P. pastoris transformed

with an empty vector.

Recombinant Gal-5 and Gal-2 were purified from the culture medium by ion exchange

chromatography. Cation exchange chromatography using Sulphoxyethyl (SE) cellulose was

efficient in purification of these enzymes because P. pastoris culture medium contains low levels

of secreted endogenous protein, and both enzymes bind SE due to their high positive net charge

at around pH 6. It allowed 2.7-fold purification of Gal-5 and 2-fold purification of Gal-2 in a

single step. Purified Gal-5 and Gal-2 appeared on SDS –PAGE gel as single bands with an

estimated monomeric molecular weight of ~75 kD (Fig. 4A, B), indicating their purification to

near homogeneity. This result was supported by acidic native polyacrylamide gel electrophoresis

36

Fig. 4. SDS-PAGE (A), western blot (B), and zymogram (C) of purified Gal-5 and Gal-2.

A-10% SDS gel was stained with Coomassie Blue R-250. B-Purified proteins separated on 10%

SDS-PAGE were transferred to a nitrocellulose membrane, which was incubated with rabbit

anti- whole Gal-2 antiserum. C-Proteins were subjected to 8% acidic native gel and stained with

4-methylumbelliferyl galactoside. Lane MW, molecular weight standard; Lane 1, Gal-5;

Lane 2, Gal-2.

50 kD

150 kD

100 kD

75 kD

37 kD

A B C

25 kD

MW Gal-5 Gal-2 Gal-5 Gal-2 Gal-5 Gal-2

37

that showed a single activity band on the zymogram for each enzyme (Fig. 4C). It should be

noted that the monomeric molecular weight of Gal-5 and Gal-2 obtained by SDS-PAGE is

slightly lower than the estimation based on amino acid sequence (79- kD for Gal-5 and 81-kD for

Gal-2). Peptide mass fingerprinting (MALDI-TOF) analysis showed that both ends of the Gal-5

polypeptide sequence were present in the peptide mixture, which ruled out proteolytic

modification of Gal-5 during purification. Thus, the lower experimentally estimated monomeric

molecular weight of Gal-5 and Gal-2 is likely due to their high content of hydrophobic amino

acids (Gal-5 40%, Gal-2 42% compared to BSA (bovine serum albumin) 34%). Hydrophobic

amino acids bind SDS at a higher ratio compared to hydrophilic ones, thereby causing slightly

faster mobility of hydrophobic proteins on SDS-PAGE (Bayreuther et al., 1980).

The fact that Gal-5 and Gal-2 bind to a cation exchanger at pH~6 and migrate through

acidic native gels indicates that these proteins have high pI values, consistent with the predicted

pIs of 8.1 and 8.6 for Gal-5 and Gal-2, respectively. It is interesting to note that basic pI values

were also observed for kiwifruit, tomato, and carambola β-galactosidases (Ross et al., 1993;

Carey et al., 1995; Balasubramaniam et al., 2005). These results support the hypothesis that the

cell wall-associated proteins interact ionically with the acidic cell wall matrix. Pectic

polygalacturonic acid provides negative charges and contributes to the interaction with high pI

proteins (Jamet et al., 2006).

2.6. Properties of recombinant Gal-5 and Gal-2

Recombinant Gal-5 and Gal-2 displayed optimal activity on pNP-β-D-galactopyranoside

between pH 4.0 and 4.5. The pH-activity profile had a narrow range in that less than 50% of

38

maximal activity was observed at pH values below 3.5 and above 6.0. These data are consistent

with the pH optima of most of the well-characterized plant β-galactosidases, which range from

3.0 to 5.0 (Carey and et al, 1995; Cicek, 1998; Edwards Mary, 1988; Gross, 2001; Kotake et al,

2005). It is generally believed that the pH of the cell wall is acidic, around 5.5. During growth

and expansion of the cell, the pH of the cell wall falls below 5.5 (Grebe, 2005), which is optimal

for the activity of most cell wall modifying enzymes. When β-galactosidase activity was

measured as a function of temperature, both Gal-5 and Gal-2 showed temperature optima at 40

°C and were stable up to 55 °C (data not shown). At temperatures above 55 °C, both enzymes

were inactivated irreversibly.

2.7. Aglycone, glycone and natural substrate specificity

To determine the specificity of the enzymes for the aglycone moiety, β-galactosides with

a variety of aglycones such as ortho-nitrophenyl (oNP), para-nitrophenyl (pNP), 4-

methylumbelliferyl (4MU), 5-bromo-4-chloro-3-indolyl (X) and 6-bromo-2-naphthyl (6BN)

were tested. All of them were hydrolyzed by both Gal-5 and Gal-2, albeit with different

efficiencies (Table 2). These results indicate that they have a broad specificity with respect to the

aglycone moiety. To investigate the specificity for the glycone moiety, pNP-β-D-

galactopyranoside (pNPGal), pNP−β-D-mannopyranoside, pNP−β-D-fucopyranoside, pNP−β-D-

xylofuranoside, pNP−β-D-arabinopyranoside, and pNP−α-L-arabinopyranoside (pNPAra) were

tested. The results are summarized in Table 3, which show that only pNP-β-D-galactopyranoside

and its 6-deoxy analogue pNP−β-D-fucopyranoside were hydrolyzed. Gal-5 and Gal-2 failed to

hydrolyze pNPAra, showing that the glycone specificity of these two enzymes is strict. Thus,

39

Table 2. Aglycone specificities of Gal-5 and Gal-2

Relative activitya, % Aglycone

Gal-5 Gal-2 para- nitrophenyl- (pNPGal) 100 100 ortho-nitrophenyl- (oNPGal) 61 72 4-methylumbelliferyl- (4MUGal) 18 20 5-bromo-4-chloro-3-indolyl- (X-Gal) 43 56 6-bromonaphthyl- (6BNGal) 22 30

aActivities of Gal-5 and Gal-2 were assayed in reaction mixtures containing 2.5 mM substrate in NaOAc buffer pH 4.6. Aglycone specificity is expressed as a percentage of activity against pNPGal (100% ~ 0.03 units (nkat)). For insoluble aglycones, amounts of galactose produced as a result of hydrolysis were measured by the galactose dehydrogenase assay.

40

Table 3. Sugar specificities of Gal-5 and Gal-2

Relative activitya, % Glycone Gal-5 Gal-2

pNP-β-D-galactopyranoside 100 100 pNP-β-D-fucopyranoside 25 21 pNP-β-D-glucopyranoside <1 <1 pNP-β-D-mannopyranoside 0 0 pNP-β-D-xylofuranoside 0 0 pNP-β-D-arabinopyranoside 0 0 pNP-α-L-arabinopyranoside 0 0 aActivities of Gal-5 and Gal-2 were assayed in reaction mixtures containing 2.5 mM substrate in NaOAc buffer pH 4.6. Sugar specificity is expressed as a percentage of activity against pNPGal (100% ~ 0.03 unit (nkat)).

41

Gal-5 and Gal-2 are highly specific for β-galactopyranoside and discriminate sugars based on the

configuration of the hydroxyl group at C4 and C3 positions.

Kinetic parameters of Gal-5 and Gal-2 were determined with pNP-β-D-

galactopyranoside. Km values for pNP-β-D-galactopyranoside for the two enzymes were similar

(0.28±0.06 mM for Gal-5 and 0.40±0.02 mM for Gal-2), but kcat values were different (1.55 s-1

for Gal-5 and 6.03 s-1 for Gal-2). Their catalytic efficiencies (kcat/Km) differed to some extent

(5.54 s-1mM-1 for Gal-5 and 15.21 s-1mM-1 for Gal-2). Km values for oNPGal and pNPFuc were

0.83±0.015 and 3.75±0.55 mM for Gal-5, respectively, and 0.72±.016 and 6.4±2.8 mM for Gal-

2, respectively.

Inhibitory effects of several sugars and sugar derivatives were tested using pNPGal as a

substrate. γ-Galactonelactone and D-galactose were the most effective inhibitors for Gal-5 and

Gal-2 activity. Their Ki values were 44 µM and 7.4 mM, respectively for Gal-5 and 98 µM and

4.5 mM for Gal-2. Also D-fucose, methyl-α-D-galactoside and raffinose were weaker inhibitors

for both enzymes while pNPGlc, pNPAra, lactose, IPTG, galacturonic acid, L-arabinose, and D-

mannose did not show any inhibitory effects. Ag+, Hg2+ and SDS strongly inhibited activity of

both enzymes when pNPGal was used as a substrate.

β-galactosidases from different plants or within the same plant are known to differ

considerably in their linkage specificity (Kotake et al., 2004; Ishimura et al., 2005; Buckeridge et

al., 2005). Using β-(1, 4), β-(1, 3) and β-(1, 6) linked galacto-oligosaccharides, we investigated

the linkage specificity of Gal-5 and Gal-2. As can be seen from Fig. 5, both Gal-5 and Gal-2

hydrolyze β-(1, 4) (lanes 3-4 and 5-6) and β-(1, 3) (lanes 8-9 and 10-11) linkages, whereas the β-

(1,6) linkages in galacto-oligosaccharides were less susceptible to hydrolysis (lanes 13-16).

42

Fig. 5. Linkage specificity of Gal-5 and Gal-2. After hydrolysis of galactobioses and

galactotrioses by Gal-5 and Gal-2, products were separated by TLC and developed with

naphthoresorcinol (see methods). Lane 1, monogalactose, Lane 2- β−(1, 4)- linked galactobiose

and galactotriose; Lane 3 and 4, hydrolysis product of β−(1, 4)- linked galactobiose and

galactotriose by Gal-5; Lane 5 and 6, hydrolysis product of β−(1, 4)- linked galactobiose and

galactotriose by Gal-2; Lane 7, β−(1, 3)- linked galactobiose and galactotriose; Lane 8 and 9,

hydrolysis product of β−(1, 3)- linked galactobiose and galactotriose by Gal-5; Lane 10 and 11,

hydrolysis product of β−(1, 3)-linked galactobiose and galactotriose by Gal-2; Lane 12, β−(1,

6)- linked galactobiose and galactotriose; Lane 13 and 14, hydrolysis product of β−(1, 6)- linked

galactobiose and galactotriose by Gal-5; Lane 15 and 16, hydrolysis product of β−(1, 6)-linked

galactobiose and galactotriose by Gal-2.

43

Ahn et al. (2007) showed that a member of the family, Gal-4, preferentially cleaves β-(1, 4) and

β-(1, 3) linkages. Thus, the three Arabidopsis paralogs, Gal-5, Gal-2, and Gal-4, might act on the

same natural substrates with β-(1, 4) and β-(1, 3) linkages. To probe the natural substrate

specificity of Gal-5 and Gal-2, more complex oligo-/polysaccharides were tested and the results

are shown in Table 4. L-Arafase (α-L-arabinofuranosidase) pretreated (to remove arabinose)

lupin galactan, a polymer of β-(1, 4) linked galactose, was hydrolyzed to some extent, whereas

gum arabic and gum guar were not hydrolyzed. Commercially prepared apple pectin (Sigma,

P8471) was the best substrate among the polysaccharides tested. In the case of larchwood

arabinogalactan and gum arabic galactan, we were unable to measure hydrolysis, since the

negative controls had high background due to the presence of components that interfere with the

galactose dehydrogenase assay (#2570-050, Interscientific, Hollywood FL). The fact that Gal-5

and Gal-2 hydrolyzed lupin galactan suggests that these enzymes have exo-galactanase activity.

Exo-galactanase activity has been reported for lupin β-galactosidase (Buckeridge et al., 1994 and

Buckeridge et al., 2005), a tomato β-galactosidase (TBG4) (Carey et al. and 1995; Ishimaru et al.

2005;), and apple β-galactosidase (Ross et al., 1994).

The strict specificity of Gal-5 and Gal-2 for galactose and their ability to hydrolyze β-(1,

4) and β-(1, 3) linkages in galactooligosaccharides and β-(1, 4) linkages in lupin galactan suggest

that cell wall polysaccharides rich in galactan are more likely to be natural substrates for these

enzymes. Rhamnogalacturonan-I (RG-I) and rhamnogalacturonan-II (RG-II), the pectic

polysaccharides of Arabidopsis cell wall, contain side chains rich in terminal galactose residues

with β-(1, 4) and β-(1, 3) linkages (Zablackis et al, 1995). Besides pectins, xyloglucans from

hemicellulose are also known to have terminal β-(1, 2) linked galactose residues that are

susceptible to cleavage by β-galactosidases. Interestingly, xyloglucan oligosaccharides were

44

resistant to the action of Gal-5 and Gal-2. Both enzymes released galactose only from the hot

water-soluble and ammonium oxalate-soluble pectic fractions of Arabidopsis cell wall, though at

a slow rate (Table 4). Gal-5 and Gal-2 (~0.03nkat) released ~1 µg of galactose from 2 mg of

Arabidopsis cell wall. The crude extract from rosette leaves released even less galactose under

the same conditions. This observation is likely due to the complex structure of cell wall

polysaccharides (e.g. RG-I) that contain side chains with linear and branched α-L-arabinose and

β-D-galactose residues that sometimes can be substituted by α-L-fucose, β-D-glucuronic acid,

and 4-O-methyl-β-D-glucuronic acid residues. Such substitutions make galactose residues

inaccessible to the action of β-galactosidases and hence limit hydrolysis (Zablackis et al., 1995,

Kotake et al. 2005, Iglesias et al., 2006). It should be noted that the limited action of Gal-5 and

Gal-2 on the side chains of the pectic backbone can increase porosity of the matrix that creates

microenvironments in cell walls in vivo, which in turn may control accessibility of other cell

wall-degrading enzymes to their substrates (Smith et al., 2002, Verbelen and Vissenberg, 2007).

Conclusions

Most of the enzymes and structural proteins that are directly involved in construction and

functioning of Arabidopsis cell wall are encoded by multigene families (Farrokhi et al., 2006).

These families consist of members sharing structural similarity, but differing in their temporal

and spatial expression profiles and physiological functions. GH family 35 enzymes, consisting of

17 putative β-galactosidases in Arabidopsis, are believed to be involved in cell wall dynamics

(Imoto et al., 2005, Ahn et al., 2007, Verbelen and Vissenberg, 2007). We studied two members

45

Table 4. Natural substrate specificities of Gal-5 and Gal-2.

Activitya Substrate Linkage of terminal residue Gal-5 Gal-2 Crude

Arabidopsis cell wall, hot water soluble pectin Unknown 0.7 0.7 0.3

Arabidopsis cell wall, oxalate soluble pectin Unknown 0.6 1.3 0.6

Arabidopsis cell wall, alkali soluble hemicellulose I Unknown tr 0.7 0.6

Arabidopsis cell wall, alkali soluble hemicellulose II Unknown tr tr tr

Lupin galactan Galβ−(1, 4)Gal 1.7 2.5 N/A

Apple pectin unknown 0.8 4.5 1.5

Gum Arabic Galβ−(1, 3), (1, 6) Gal 0 0 N/A

Gum Guar Manβ-(1, 4)Man, Galα−(1, 6)Gal 0 0 N/A a- µg of Gal released from 2 mg of polysaccharide by 0.03 nkat (units) of enzyme at 25oC in 24 h, tr- trace (poor hydrolysis), N/A-not tested

46

of the Arabidopsis β-galactosidase family, Gal-5 and Gal-2. Western blot analysis using peptide-

specific antibodies revealed organ-specific expression of the two genes encoding these enzymes.

We showed that Gal-5 and Gal-2 are present in and tightly associated with the cell wall in

Arabidopsis by using peptide-specific antisera and dot blotting. Recombinant Gal-5 and Gal-2

expressed in P. pastoris hydrolyzed various synthetic galactosidases, galacto-oligosaccharides

and cell wall-derived polysaccharides. Both enzymes preferentially cleaved galactosides

containing β-(1, 4) and β-(1, 3) linkages. The properties of the enzymes and their natural

substrate specificities suggest that they may have the potential to be involved in modification of

pectic polysaccharides of cell wall matrices. Further studies are needed to understand their

biological roles as pectin-modifying enzymes.

3. Experimental

3.1. Materials

cDNAs for Gal-5 (pda05881 or pda06378) and Gal-2 (pda01770) in pBluescript vector

were obtained from RIKEN, Institute of Physical and Chemical Research, Japan. Enzymes for

the cloning were from Stratagene (La Jolla, CA) and NEB (Ipswich, MA). Easy select Pichia

expression kit was from Invitrogen (Carlsbad, CA). Synthetic substrates and other chemicals

were from Sigma (St. Louis, Mo). The imject maleimide activated BSA conjugation kit was from

Pierce (Rockford, IL). Total Galactose Neonatal Screening Test Kit was from Interscientific

(Hollywood, FL). Galactose dehydrogenase was from Roche (Indianapolis, IN). Galacto-

oligosaccharides were a gift from Dr. Yoichi Tsumuraya and Dr. Toshihisa Kotake of Saitama

University, Japan. Lupin galactan was a gift from Dr. David Smith of USDA, Beltsville, MD.

47

3.2. Methods

Database analysis

Signal sequence predictions and subcellular targeting of predicted proteins were done

using SignalP (Bendtsen et al., 2004) and TargetP (Emanuelsson et al., 2000). Amino acid

sequences of β-galactosidases were aligned using software at http://align.genome.jp. A

phylogenetic tree was constructed from the alignment using PAUP 4.0.

Microarray expression analysis

Expression profile analysis was done using Arabidopsis gene expression datasets from

the Genevestigator website (http://www.genevestigator.ethz.ch). Using Gene Atlas Tool, the

organ/tissue-specific expression levels for At1g45130 (Gal-5) and At3g52840 (Gal-2) were

estimated by Genevestigator software along with Atg05320 (Ubiquitin10, UBQ10) as the control

for comparison. A given gene was scored as “expressed” if data from the Digital Northern Tool

gave signal values higher than 200 with p<0.06 (Zimmermann, 2004). The p value for Gal-5 and

Gal-2 was p=0.00164. Response Viewer Tool was used to verify up- and down- regulated genes

under different abiotic and biotic stresses. Reliability and reproducibility of analyses were

evaluated by the number of chips and replicates in individual experiments.

Plant materials

Arabidopsis seeds (Col-O) were obtained from the Arabidopsis Biological Resource

Center (ABCR), Ohio State University Seed Stock Center (Columbus, OH). For germination,

seeds were surface-sterilized with 3% hypochlorite for 10 minutes followed by washes with

48

dH2O (three times) and finally suspended in 0.1% agarose. Sterilized seeds were kept at 4oC for

3-4 days and seedlings were germinated on half strength Murashige-Skoog salt-agar plates for

10-14 days with 16 h day and 8 h night cycles. Seedlings were transferred to soil and grown at

16/8 h day/night cycle. Plants were harvested when 4-5 weeks old, and immediately frozen in

liquid nitrogen and kept at -80oC until use. For western blot analysis, Arabidopsis tissue was

ground with sand (0.3 g/1 g tissue). Total proteins were solubilized in 6M urea (1g tissue: 2 ml

solvent). Cell wall polysaccharide isolation was done as described in Li et al. (2001).

Expression of Gal-2 in E. coli and preparation of rabbit antisera

The Gal-2 mature protein coding sequence was cloned into pET21a vector and expressed in E.

coli BL21 codon plus cells. Cells were suspended in lysis buffer (50 mM Tris-HCl pH 8.0, 100

mM NaCl, 0.02% SDS, 1 mM PMSF) and broken up using a French press. After extensive

washing of soluble fractions with lysis buffer, insoluble proteins (the inclusion body fraction)

were solubilized in 6 M urea and separated on a 10% SDS-PAGE preparative gel. The gel was

stained (30 min) with Coomassie brilliant blue R-250 and the band corresponding to the Gal-2

polypeptide was excised. The excised band was destained in 50% methanol with several changes

of solution and rehydrated in a minimum amount of 1X PBS at 4oC overnight. After rehydration,

the band was ground in a pre-chilled mortar. Ground powder was suspended in 1X PBS

containing 0.2% SDS and 0.5% 2-mercaptoethanol (v/v) and heated at 75oC for 15 min. After

cooling, the suspension was mixed with 1 volume of Freund’s Complete Adjuvant (Sigma) and

used for immunization. Rabbit anti-Gal-2 sera were raised by repeated injection of antigen mixed

with Freund’s Incomplete Adjuvant at two-week intervals. Synthetic peptides (Gal-2:

CSGKIRAPTILMKMIPTS and Gal-5: CSGVAFLTNYHMNAPAKVV) were conjugated to the

49

BSA using an imject maleimide activated BSA kit (Pierce, Rockford, IL) according to the

vendor’s protocol. Peptide-specific antisera were raised by injecting BSA-conjugated synthetic

peptides with Freund’s adjuvants. A small volume of trial bleeding was taken at two-week

intervals to monitor the change in antisera titer during the course of immunization. All antisera

were diluted twice with glycerol and stored at -20oC until usage.

Expression of recombinant Gal-5 and Gal-2 in P. pastoris and purification

The mature protein coding sequences of Gal-5 (S24 through N724) and Gal-2 (V28

through K727) cDNAs were amplified by the primer pair 5’-CAC CGT GGT CAC TTA TGA

TCA CAA AGC-3’ and 5’-CCA ATG AAA GAG GGT AAC AAA GGGC-3’ for Gal-2 and 5’-

AGG TGA ATT CCA GTG TAG TAG TGT AAC CTA CG-3’ and 5’-TTT GCG GCC GCA

AGT TAG TTT ACT GAT CTC TTC ACA AC-3’ for Gal-5 from cDNA inserts of plasmids

obtained from RIKEN, using the high-fidelity Pfu Turbo DNA polymerase. The inserts were

cloned into pPICZα Pichia expression vector to express Gal-5 and Gal-2 as the yeast α factor

secretion signal fusion protein to facilitate secretion of recombinant proteins into culture

medium. After confirming the accuracy of the sequence and the correct reading frame, linearized

plasmids were transformed into P. pastoris by electroporation. Recombinant enzyme production

was under the control of the alcohol oxidase (AOX) promoter induced by methanol. Production

of recombinant proteins was monitored by assaying β-galactosidase activity toward pNPGal in

culture supernatant samples taken every 24 h. After 72 hours of induction, cells were pelleted

and culture supernatant was used for further purification of the recombinant enzymes.

Gal-5 and Gal-2 were purified from culture supernatants by ion exchange

chromatography using Sulphoxyethyl (SE) cellulose. In a typical experiment, 100 ml of culture

50

supernatant was filtered, diluted five times with degassed dH2O to reduce ionic strength of the

medium and loaded onto the column (1.5 cm x 3 cm) pre-equilibrated with buffer A (20 mM

potassium phosphate, pH 6.0). After washing with 10 column volumes of buffer A, bound

proteins were eluted in one step with 150 mM NaCl in buffer A (flow rate 1ml/min). All

fractions were checked for β−galactosidase activity using pNPGal as substrate. Fractions with

highest β-galactosidase activity were used for further experiments. Protein concentration was

determined by the Bradford method (Bio-Rad Protein Assay Reagent kit) using BSA as a

standard.

Cell wall isolation and extraction of cell wall-bound proteins

We isolated cell wall from rosette leaves of Arabidopsis using the procedure described by

Feiz et al. (2006). Rosette leaves were ground in a blender whose cup was dipped at intervals

into liquid nitrogen to maintain low temperature during grinding. The cell wall fraction was

washed extensively with 3 L of wash buffer on a metal net (75 µm pore size). After washing, the

cell wall fraction was lyophilized. The lyophilized cell wall material was ground to a fine powder

by grinding in a blender, and was then used to extract wall-bound proteins. Five different (S1,

soluble 1; S2, soluble 2; S3, soluble 3; CW4, extractable with CaCl2; and CW5, extractable with

LiCl) fractions were obtained. These fractions were assayed for β-galactosidase activity. Of

these, three fractions with β-galactosidase activity were further analyzed for immunoreactivity.

They were spotted multiple times on nitrocellulose strips to increase antigen (Gal-5 and Gal-2)

concentration and incubated with preimmune (control) and immune sera from rabbits immunized

with whole Gal-2 polypeptide and unique peptides derived from Gal-5 and Gal-2 sequences.

51

SDS-PAGE, native PAGE and western blotting

SDS-PAGE was performed as described by Laemmli (1970). Native PAGE was

performed in acidic gels using the protocol on the website

(http://wolfson.huji.ac.il/purification/Protocols/PAGE_Acidic.html). After electrophoresis, the

gel was rinsed in a wash buffer (100 mM acetate buffer pH 4.6) for 2x15 min. The zymogram

was developed by incubating the gel in 0.5 mM 4-MUGal in wash buffer at 37oC for 20 min and

photographed under UV light. For immunoblotting, the gel was soaked in a blotting buffer (10

mM CAPS, pH 11 with 10 % (v/v) methanol) for 2x15 min. Proteins were transferred onto a

nitrocellulose (0.45 µm, Protran) membrane using a Bio-Rad Mini trans blot cell at 50V, at 4oC

overnight following the vendor’s protocol. For immunodetection, 2000-times dilution of anti-

Gal-2 antiserum or 1000-times dilution of peptide-specific antiserum was used as primary

antibody and 2000-times dilution of goat anti-rabbit antibody conjugated with peroxidase

(A0545, Sigma, Saint Louis, MO) as secondary antibody. Immunoreactive bands were visualized

by the deposition of 4-chloronaphthol after oxidation by HRP (horse radish peroxidase) using the

substrate solution (21 ml of PBS pH 7.4, mixed with 5.5 ml of 3.3 mg/ml 4-chloronaphthol in

100% MeOH and 10 µl of 30% H2O2).

β-galactosidase activity assay

One hundred µl of 5 mM pNPGal in 100 mM NaOAc buffer pH 4.6, 80 µl H2O and 20 µl

of the diluted enzyme solution were mixed and incubated at 37oC for up to 30 minutes. The

reaction was stopped by adding 100 µl of 1M Na2CO3. The absorbance was measured at 405 nm

to quantify the amount of pNP released after hydrolysis. This standard protocol was used for all

activity assays with pNPGal, if not otherwise stated. Boiled enzyme or buffer solution was used

52

as a control. One unit (nkat) of enzyme activity is defined as an amount of enzyme that is able to

produce one nmole of pNP per second at 37oC.

For the determination of natural substrate specificity, galacto-oligosaccharides (20 mM),

xyloglucan oligosaccharides (1 µg/µl) and polysaccharides (1% (w/v)) were prepared in water.

Final concentration of substrates was 4 mM for oligosaccharides and 0.5% for polysaccharides in

100 mM acetate buffer pH 4.6. Reaction mixture was incubated with 0.03 units/nkats enzymes at

room temperature for 24 h. Reaction mixtures containing no enzyme and no substrate were used

as controls. Reactions for polysaccharides were stopped by adding 1 ml of 100% EtOH to 0.4 ml

of reaction mix to precipitate proteins and polysaccharides. After centrifugation, the supernatant

was transferred into a new microfuge tube and vacuum dried. Dried mixtures were dissolved in

100 µl of dH2O, and total galactose produced as a result of hydrolysis was quantified using

galactose dehydrogenase assay kit (Interscientific, Hollywood, FL). Products of hydrolysis of

oligosaccharides were analyzed by thin layer chromatography (TLC) on silica gel 60F254 (EM

Science, Germany) using 3:2:1 (v/v/v) butanol: acetic acid: water as the solvent and detected by

heating TLC plates after spraying with 0.2% (w/v) naphthoresorcinol in 1:19 H2SO4: ethanol

(v/v) (Ahn Young Ock, 2004)

53

Acknowledgements

We are grateful to Drs. Yoichi Tsumuraya and Dr. Toshihisa Kotake of Saitama

University, Japan for providing galacto-oligosaccharides, and Dr. David Smith of USDA,

Beltsville, MD for providing lupin galactan. The authors also wish to thank Dr. Farooqahmed

Kittur of Virginia Tech for much help and critical reading of the manuscript. This research is

funded by the Arabidopsis 2010 Project of the National Science Foundation (MCB-0115937) and

a research grant award from the Virginia Academy of Science to D. Gantulga.

54

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60

Chapter 3

Comparative characterization of the Arabidopsis Subfamily III β-galactosidases

Keywords: Arabidopsis; Cell wall; Glycosyl hydrolase family 35; Subfamily ΙΙΙ, β-

Galactosidase; cell wall modification;

Abbreviations: Arabidopsis thaliana, Arabidopsis; Gal-1, (AtBGal1)–β−galactosidase-1,

Gal-2, (AtBGal2)–β−galactosidase-2, Gal-3, (AtBGal3)–β−galactosidase-3, Gal-4,

(AtBGal4)–β−galactosidase-4, Gal-4, (AtBGal5)–β−galactosidase-5; Gal-12,

(AtBGal12)–β-galactosidase-12; pNP, para-nitrophenyl; GH, glycosyl hydrolase.

61

Abstract

The Arabidopsis genome encodes 17 predicted β−galactosidases that have been

classified into seven subfamilies based on sequence similarity. The largest of these,

Subfamily III, consists of six genes, Gal-1 (At3g13750), Gal-2 (At3g52840), Gal-3

(At4g36360), Gal-4 (At5g56870), Gal-5 (At1g45130), and Gal-12 (At4g26140), several

of which were characterized in previous studies. We report here the purification and

biochemical characterization of recombinant Gal-1, Gal-3, Gal-4 and Gal-12 from Pichia

pastoris, completing the analysis of all six recombinant proteins, as well as the isolation

and characterization of the native Gal-2 protein from Arabidopsis leaves. Comparison of

the relative expression levels of the six Gal genes uncovered evidence of differential

regulation. In addition, this study provides further support for the proposed function of

the Subfamily III β−galactosidases in cell wall modification based on organ-specific

expression by immunoblotting and subcellular localization by immunoEM of Gal-1 and

Gal-12. Our study suggests that despite small differences in individual biochemical

characteristics and expression patterns, each member of the family has the potential to

contribute to cell wall dynamics.

62

1. Introduction

β-galactosidases (EC 3.2.1.23) are enzymes that cleave substrates containing

galactosyl moieties, such as lactose, glycolipids, proteoglycans, oligosaccharides, and

polysaccharides. β-galactosidases are of interest because of their use in the dairy and food

industry, agriculture, and biotechnology (Callahan, 1999; Smith and Gross, 2000;

Tanthanuch et al., 2008). Based on amino acid sequence similarities, β-galactosidases

have been found to fall into four of the 113 current glycosyl hydrolase (GH) families, GH-

1, GH-2, GH-35 and GH-42 (Coutinho and Henrissat, 1999; http://www.cazy.org/). β-

galactosidases belonging to GH-1, GH-2, and GH-42 are found predominantly in

microorganisms, whereas the GH-35 enzymes are found in both prokaryotes and

eukaryotes. In plants the β-galactosidases belonging to GH-35 family constitute gene

subfamilies with high sequence similarities, where in other eukaryotes the total number of

β-galactosidase genes is much smaller.

Plant β-galactosidases play important roles in the metabolism of galactosyl

conjugates during carbohydrate reserve mobilization, cell wall expansion and degradation,

and turnover of signaling molecules (de Alcantara et al., 2006; Esteban et al., 2003;

McDougall and Fry, 1990). A correlation between β-galactosidase expression and cell

wall disassembly has been reported in fruit ripening of tomato (Carey et al., 1995), apple

(Ross et al., 1994), kiwi (Ross et al., 1993), pear (Tateishi et al., 2001), and papaya (Lazan

et al., 2004). Besides fruit ripening, β-galactosidases have been shown to participate in

seed germination in Arabidopsis (Dean et al., 2007), radish (Kotake et al., 2005; Sekimata

et al., 1989), and rice (Chantarangsee et al., 2007). β-galactosidase activities associated

with various physiological processes have been observed in different plant tissues,

63

including the cotyledon of lupin (Buckeridge et al., 2005), floral tissue of Sandersonia

flower (O'Donoghue et al., 2002), seedling of mung bean (Li et al., 2001), and pollen of

tobacco (Hruba et al., 2005). The analysis of individual β-galactosidases purified from

different plants has led to better understanding of their functions and natural substrate

specificities. Further genetic and bioinformatics studies are necessary to understand the

interrelationships among β-galactosidase isoforms and the mechanisms by which they

participate in the regulation of growth and development.

For a systematic characterization of β-galactosidases, the most attractive organism

is a model plant with a completely sequenced genome. In Arabidopsis thaliana, a gene

family of 17 β-galactosidases has been identified (Ahn et al., 2007). Among these, the

members of Subfamily III, which consists of Gal-1, Gal-2, Gal-3, Gal-4, Gal-5 and Gal-

12, is the largest subfamily. The presence of such a large subfamily raises the question of

functional redundancy of these isoforms in plants. Many homologs of Subfamily III genes

have been studied in other plant species (Buckeridge et al., 2005; Carey et al., 1995;

Chantarangsee et al., 2007; Lazan et al., 2004; Tateishi et al., 2001; Trainotti et al., 2001),

but a characterization of the entire subfamily from a single species has not yet been carried

out. We recently reported the biochemical characterization of Gal-4 (Ahn et al., 2007) and

Gal-2 and Gal-5 (Gantulga et al., 2008), showing that these enzymes have high hydrolase

activity with specific cell wall polysaccharides. We also demonstrated the presence of

Gal-2 and Gal-5 proteins in the cell wall of rosette leaves by immuno dot-blotting. Our

results suggested that the sequence similarities of the genes in the subfamily translate into

functional similarities of the proteins in cell wall modification.

64

In this work we extend the previous analysis of Gal-2, Gal-4, and Gal-5 to

complete the biochemical characterization of the Subfamily III β-galactosidases. We also

describe the purification and characterization of a native β-galactosidase from this

subfamily from Arabidopsis leaves. Detailed organ-specific expression of all six genes in

the subfamily was studied by semi-quantative RT-PCR and compared with the publicly-

available microarray data. We show that methyl jasmonate treatment, known to arrest

growth and control defense responses in plants, down-regulates the expression of the Gal

genes. Immunolocalization of Gal proteins using transmission electron microscopy

(TEM) of roots confirmed the predicted localization of the Subfamily III galactosidases in

the cell wall in vascular and epidermal tissues of mature root.

2. Results and discussion

2.1 In silico characterization of the Arabidopsis Subfamily III β-galactosidases

Previous analysis of the evolutionary relationships among the GH Family 35 β-

galactosidase genes in 23 eukaryotic genomes showed that the 17 Arabidopsis genes fall

into two groups and seven subfamilies (Ahn et al., 2007). Fig. 1 shows an unrooted tree

based on the amino acid sequences predicted for these 17 genes. Gal-1, Gal-2, Gal-3, Gal-

4, Gal-5, and Gal-12 comprise the largest subfamily, designated as Subfamily III

(Gantulga et al., 2008). These six proteins share 60-81% sequence identity with each

other. The other subfamilies each consist of one to three members. All six members of

Subfamily III are predicted to have signal peptides and basic pIs (7.2-8.6), consistent with

a cellular destination in the cell wall (Ahn et al., 2007). This localization was recently

65

Fig. 1. Phylogenetic relationships among the Arabidopsis thaliana β-galactosidases.

The tree of the β−galactosidase family is derived from an alignment of 17 members that

are divided into 7 subfamilies based on the amino acid sequence.

66

confirmed for Gal-2 and Gal-5 (Gantulga et al., 2008). The six enzymes differ with regard

to the presence of a C-terminal lectin-like domain in Gal-1 and Gal-3, which places these

two isoforms with the higher molecular mass Group 2 β-galactosidases (Ahn et al., 2007).

It has been suggested that the lectin-like domains may enhance catalytic efficacy by

anchoring the enzymes to their polymeric substrates.

2.2 Expression of β-galactosidases in Arabidopsis

As a first approach to characterizing the relative expression patterns of the

Subfamily III genes, the publicly available microarray data were examined using

Genevestigator (Zimmermann et al., 2004). We previously used this database to

determine that Gal-2 and Gal-5 exhibit differential, tissue-specific expression patterns

(Gantulga et al., 2008). Extending this analysis to include all six members of Subfamily

III showed that the genes are expressed in all tissues examined, but to different degrees

(Fig. 2). Gal-1 was the most abundantly expressed of the six genes in most tissues, with

the highest levels in the petiole, stem, and cauline leaf and moderate levels in all other

organs. Gal-3 had the highest expression in the stem and flowers, while Gal-4 transcripts

were most abundant in silique, stem, root and cauline leaves. The relative levels of the

Gal-12 transcripts were low in almost all organs except root, specifically root hairs.

To confirm these findings and to extend the analysis of expression patterns to

include such organs as mature root, cauline leaf, petiole of the rosette leaf, and silique,

semi-quantitative RT-PCR was performed. These experiments provided further evidence

for temporal and spatial differences in the expression of the six Subfamily III genes (Fig.

3A). Most of the genes are expressed in all organs, although the relative abundance of the

67

Fig. 2. Expression profile of the Subfamily III β-galactosidases from the public

microarray data. The organ/tissue-specific expression levels for Gal-1, Gal-2, Gal-3,

Gal-4, Gal-5, and Gal-12 are shown as log2(n) based on data obtained using

Genevestigator {Zimmermann, 2004 #4}. Data for ubiquitin-10 (UBQ10) was included

for comparison. The Gene Atlas Tool of Genevestigator

(https://www.genevestigator.ethz.ch/at/) was used for the analysis. Numbers on the Y axis

represent the average signal intensity.

68

transcripts varied. Only three of the genes, Gal-1, Gal-3, and Gal-4, were expressed in

siliques. Gal-1 transcripts were detected in all organs: root, leaf, stem, flower, siliques,

and young seedling, Fig. 3A. The Gal-1 transcript level was the highest in root followed

by leaves and flower. The Gal-2 transcript was the most abundant in young seedling and

was lowest in root and flower. The Gal-3 transcript was highest in stem and silique, but

was low in all other organs examined. The Gal-4 transcript was abundant in root, stem

and flower, but was low in leaves. The Gal-5 and Gal-12 transcripts were detected at the

highest levels in green tissues and were lowest in roots. We detected two bands for the

Gal-12 RT-PCR product. This is consistent with the gene model for Gal-12 in TAIR,

which reflects alternative splicing resulting in products that differ in size by 100 bp. In

summary, the Gal-2, Gal-5, Gal-12 genes are expressed predominantly early in

development of the Arabidopsis plant, while Gal-3 and Gal-4 are expressed later in

development. In contrast, Gal-1 gene expression was similar in all stages and tissues

examined (Fig. 2).

We investigated the effects of a variety of stress conditions and hormone

treatments known to regulate plant growth and development (Nemhauser et al., 2006) on

expression of the Subfamily III genes in seedlings. Little or no difference was observed in

the transcript levels of the six genes in response to a variety of treatments, including salt,

cold, osmotic shock, or treatment with salicylic acid, 2,4-D, ACC, and ABA (data not

shown). This observation is consistent with publicly available microarray data

(Genevestigator) where the change in expression of these genes in response to similar

treatments was not significant compared to the cases of top-ranked, differentially-

regulated genes. However, notably reduced transcript levels were found for all six genes

69

A) B)

Fig. 3. Semiquantitative RT-PCR analysis of Subfamily III gene expression. A, organ

specific expression in five-week-old plant tissues; B, time course of the response to

jasmonic acid in ten-day-old seedlings. Relative expression levels of the six

β−galactosidase genes are shown relative to the ubiquitin-10 (UBQ) gene.

70

in seedlings treated with methyl jasmonate. Analysis of the time course of the response to

methyl jasmonate showed that expression of Gal-1, Gal-2, Gal-5 and Gal-12 decreased

notably only after 3h of treatment, indicating that these are late-response genes (Fig. 3B).

The transcript levels returned to pre-treatment level within 24 h. Under our experimental

conditions, expression of Gal-3 and Gal-4 was not affected as dramatically as the other

four genes. As jasmonate is known to modulate pathways controlling cell growth of

Arabidopsis (Liu and Mehdy, 2007; Staswick et al., 1992), these data provide further

support to the hypothesis that the Subfamily III β-galactosidases are involved in growth

control.

2.3 Expression and purification of recombinant Gal-1, Gal-3, Gal-4 and Gal-12 in P.

pastoris

We recently reported the biochemical characterization of recombinant Gal-2 and

Gal-5 expressed in Pichia pastoris (Gantulga et al., 2008) and of Gal-4 expressed in E.

coli and insect cells (Ahn et al., 2007) . In order to facilitate direct comparison of the

biochemical characteristics of the six Subfamily III enzymes, Gal-1, Gal-3, Gal-4, and

Gal-12 were also expressed in P. pastoris. The mature proteins without signal peptides

were targeted for secretion into the culture medium, as had been done previously for Gal-2

and Gal-5 (Gantulga et al., 2008). Gal-1 and Gal-4 were purified by cation exchange

chromatography using sulphoxyethyl cellulose (SE). Gal-3 and Gal-12 were purified by

hydrophobic interaction chromatography followed by cation exchange chromatography.

The purified proteins appeared as single bands by SDS –PAGE analysis, with estimated

monomeric molecular weights in the range of 80~90 kD (data not shown), consistent with

71

sizes predicted for these proteins, 90 kD, 92 kD, 78 kD and 79 kD, respectively for Gal-1,

Gal-3, Gal-4, and Gal-12. It is worth mentioning that substantial differences were

observed in the efficiency with which the recombinant proteins were secreted into the

culture medium. Although Gal-2, Gal-4, Gal-5, and Gal-12 were present at relatively high

levels in the medium, the levels of Gal-1 and Gal-3 were extremely low, despite the

moderate overall expression levels based on total β-galactosidase activity. The majority

of the activity remained in the cell pellet according to this assay. A modified method was

developed for purification of Gal-1 and Gal-3 from P. pastoris, starting with larger initial

culture volumes and longer induction times that substantially enhanced recovery (see

Experimental, section 3.2). It is interesting to note that Gal-1 and Gal-3 are the two

members of the subfamily with a C-terminal lectin-like domain. It is thus possible that

these enzymes are retained within the yeast cell due to interaction of this domain with the

cell wall.

2.4 Properties of recombinant β-galactosidases

The biochemical properties of the recombinant Gal-1, Gal-3, Gal-4, and Gal-12

proteins produced in P. pastoris were characterized as described previously for Gal-2 and

Gal-5 (Gantulga et al., 2008). All four enzymes showed maximum activity with pNP-

β−D-galactopyranoside at pH ~4.0 and in the temperature range of ~40-50°C (data not

shown). The enzymes were stable at temperatures up to 50oC and lost their activity

rapidly above 55oC, presumably due to heat denaturation (data not shown). The

biochemical properties and substrate specificity of Gal-4 were similar, whether the protein

was expressed in P. pastoris, E.coli, or insect cells (Ahn et al., 2007).

72

To determine the substrate preferences of the β-galactosidases for different sugar

and aglycone moieties, the ability of the recombinant Gal-1, Gal-3, Gal-4 and Gal-12

enzymes to cleave different glycoside substrates was tested using (i) pNP-β-D-

galactopyranoside (pNPGal), (ii) pNP-β-D-mannopyranoside, (iii) pNP-β-D-

fucopyranoside, (iv) pNP-β-D-xylofuranoside, (v) pNP-β-D-arabinopyranoside, (vi) pNP-

α-L-arabinopyranoside (pNPAra), (vii) oNP-β-D-galactopyranoside (oNPGal), (viii) 4-

methylumbelliferyl-β-D-galactopyranoside (4-MUGal), (ix) 5-bromo-4-chloro-3-indolyl-

β-D-galactopyranoside (X-Gal), and (x) 6-bromo-2-naphthyl-β-D-galactopyranoside (6-

BNGal). All four enzymes showed a strict specificity for galactose and its 6-deoxy

analogue, fucose (Tables I and II), indicating that only sugars with hydroxyl groups at the

C3 and C4 positions are bound and hydrolyzed by these enzymes. None of the other

glycosides that were tested were hydrolyzed by any of the family III isoforms (Table II).

In contrast to the strict sugar specificity, the enzymes showed a broad aglycone specificity.

The four enzymes hydrolyzed oNPGal, 4-MUGal, X-Gal, and 6-BNGal, although to

different extents (data not shown).

Kinetic parameters for Gal-1, Gal-3, Gal-4, and Gal-12 with several different

substrates were estimated from Lineweaver-Burk plots. The apparent Km and Vmax values

for pNPGal, pNPFuc, and oNPGal are shown in Table I. These experiments show that the

different isozymes have different Km and Vmax values with these substrates, indicating that

the enzymes have different active site requirements for substrate binding and hydrolysis.

Although no three-dimensional structures are yet available for plant β-galactosidases, the

predicted active site residues are almost identical among the six isoforms of subfamily III.

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Table I. Kinetic parametersa

pNPGal oNPGal pNPFuc Enzyme Km

mM Vmax

nkat s-1 Km mM

Vmax nkat s-1

Km mM

Vmax nkat s-1

Gal-1 3.48 (±0.11)

1324 (±67)

2.5 (±0.19)

1099 (±35)

5.34 (±1.05)

285 (±3.5)

Gal-3 0.32 (±0.06)

977 (±22)

1.9 (±0.25)

644 (±22)

5.69 (±1.9)

347 (±71)

Gal-4 1.71 (±0.13)

560 (±38)

1.28 (±0.18)

831 (±200)

3.6 (±0.8)

53.4 (±3)

Gal-12 0.71 (±0.02)

314 (±20)

4.2 (±0.4)

481 (±73)

15.8 (±0.1)

250 (±76)

nGal-2 0.7 (±0.09)

265 (±21) n/a n/a n/a n/a

aEnzyme activities were assayed in reaction mixture containing 0.1, 0.5, 1, 2.5, and 5 mM substrates in 50 mM sodium acetate buffer pH 4.6. Km and Vmax values were calculated from Lineweaver-Burk plots. Numbers represent mean ± SE (standard error) n=3 repicates. n/a- not applicable.

74

Table II. Sugar specificities of Gal-1, Gal-3, Gal-4 and Gal-12

Relative activitya, % Substrate Gal-1 Gal-3 Gal-4 Gal-12 nGal-2

pNP-β-D-galactopyranoside 100 100 100 100 100 pNP-β-D-glucopyranoside <1 <1 <1 <1 <1 pNP-β-D-mannopyranoside 0 0 0 0 n/a pNP-β-D-xylofuranoside 0 0 0 0 n/a pNP-β-D-arabinopyranoside 0 0 0 0 n/a pNP-α-L-arabinopyranoside 0 0 0 0 n/a aEnzyme activities were assayed in reaction mixtures containing 2.5 mM substrate in 50 mM NaOAc buffer pH 4.6. Sugar specificity is expressed as a percentage of activity against pNPGal (100% ~ 0.04 nkat/unit). Numbers represent mean ± SE (standard error) for N=3 replicates. n/a- not applicable.

75

A number of sugars and sugar derivatives were tested for their ability to inhibit the

hydrolysis of pNPGal by the four enzymes. The strongest inhibitors were γ-galactone-

lactone and D-galactose; the Ki values are presented in Table III. D-fucose, methyl-α-D-

galactoside, and raffinose, also inhibited hydrolysis of pNPGal by the enzymes (data not

shown). All were competitive inhibitors, leading to increased Km values for pNPGal.

Enzyme activities were not affected by the presence of pNPGlc, pNPAra, lactose, IPTG,

galacturonic acid, L-arabinose, and D-mannose in the reaction mixtures. Common non-

competitive inhibitors and denaturants were also tested for effects on the activities of these

proteins. Ag+ (silver), Hg2+ (mercury), and SDS completely abolished the activities of all

four enzymes at concentrations of 15 µM, 15 µM and 0.05%, respectively. This indicates

that these enzymes are very sensitive to these types of inhibitors.

In order to explore the action of the enzymes on natural polysaccharide substrates,

the linkage specificities of the four β-galactosidases were examined. All four enzymes

hydrolyzed β-(1, 4)- and β-(1, 3)-linked galacto-oligosaccharides, but only Gal-12 was

able to cleave the β-(1, 6) linkage in galactobiose and galactotriose (data not shown).

Together with the previous findings for Gal-2 and Gal-5 (Gantulga et al., 2008), this

indicates that Subfamily III β-galactosidases prefer β-(1, 4)- and β-(1, 3)- linkages. These

are the types of linkages found in pectic polysaccharides of the cell wall (Zablackis et al.,

1995), indicating that pectic galactans may be the natural substrates for these enzymes.

Indeed, all four enzymes hydrolyzed hot water- and oxalate-soluble pectins from

Arabidopsis cell wall and commercially prepared apple pectin, releasing sufficient

quantities of free galactose to be detectable by galactose dehydrogenase assay (Table IV),

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Table III. Inhibition coefficients

Kia

Enzyme γ-galactonelactone µM

D-galactose mM

Gal-1 267 (±32) 2.83 (±0.5) Gal-3 40 (±21) 1.0 (±0.2) Gal-4 132 (±29) 9 (±1.3) Gal-12 453 (±67) 6.3 (±0.9)

a Enzyme activities were assayed in reaction mixtures containing 0.1, 0.5, 1, 2.5, and 5 mM pNPGal in 50 mM NaOAc buffer pH 4.6 in the presence of 0, 2.5, 5, and 10 mM D-galactose or 0, 25, 50, 100 µM of γ-galactone-lactone. Ki values were calculated from a Lineweaver-Burk plot. Numbers represent mean ± SE for N=3.

77

Table IV. Natural substrate specificities

Activityb Substratea Gal-1 Gal-3 Gal-4 Gal-12 nGal-2

Hot water soluble pectin 1.7 (±0.8)

3.3 (±0.3) trb 2.3

(±0.14) tr

Oxalate soluble pectin 1.9 (±0.3) tr 1

(±0.15) 1.8

(±0.19) 1.14

(±0.2)

Alkali soluble hemicellulose I 3.4 (±0.5) tr tr tr 0.393

(±0.05)

Alkali soluble hemicellulose II 1.7 (±0.7)

1.9 (±0.9) tr tr tr

Apple pectin 13 (±2.2)

2.3 (±0.1)

4.9 (±0.8)

6.4 (±1.7)

4.5 (±0.9)

aAll polysaccharides were extracted from Arabidopsis cell wall except apple pectin, which was obtained from Sigma (St.Louis, MO). bµg of D-galactose released from 2.5 mg of polysaccharide by 0.04 nkat/units enzymes at 25oC for 30 h,. c tr - trace (poor hydrolysis) Numbers represent mean ± SE for N=3.

78

similar to what was previously shown for Gal-2 and Gal-5 (Gantulga et al., 2008) . Gal-1

and Gal-3 also had weak but detectable activities with the alkali-soluble cell wall

fractions. None of the enzymes appeared to release galactose from xyloglucan

oligosaccharides, larchwood arabinogalactan, or oat xylan (data not shown). This

observation indicates that the Subfamily III enzymes are unlikely to be involved in

modification of these types of polysaccharides in Arabidopsis.

Four of the Subfamily III enzymes, Gal-1, Gal-2, Gal-4, and Gal-12, are predicted

to have at least one N-glycosylation site. However, only two of the recombinant proteins,

Gal-1 and Gal-12, appeared to be glycosylated when expressed in yeast, based both on the

size of the recovered protein and the ability to bind to a ConA column (data not shown and

Gantulga et al., 2008). The result for Gal-4 is consistent with previous findings that this

protein is also not glycosylated when expressed in insect cells (Ahn et al., 2007). Gal-2,

Gal-3, Gal-4, Gal-5 and Gal-12 migrated through acidic native gels, consistent the ability

of these proteins to bind to cation exchange resin and the prediction that these proteins

have a basic pI based on amino acid sequence. While Gal-1 was also predicted to have a

basic pI, the recombinant Gal-1 protein did not migrate into the acidic native gels,

suggesting that it forms large aggregates in solution.

Although both Gal-1 and Gal-3 contain a C-terminal lectin-like domain, we were

not able to detect any lectin activity associated with these proteins. Neither protein bound

to galactosyl- or mannosyl-agarose columns, nor did the proteins show hemagglutinating

activity in a rabbit red blood-cell assay. The biochemical functions of these lectin-like

domains therefore remain to be elucidated.

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Taken together, these results indicate that the Arabidopsis Subfamily III β-

galactosidases are exo-galactanases that primarily act on pectic polysaccharides of the cell

wall. Therefore, these enzymes may play important roles in cell wall remodeling during

plant growth and development.

2.5 Isolation and characterization of native Gal-2 protein

Although previous microarray (Hruba et al., 2005), proteomic (Jamet et al., 2006),

and genetic (Ahn et al., 2007; Iglesias et al., 2006; Perez, 2004) studies, as well as the RT-

PCR experiments described above, all indicate that the Subfamily III β-galactosidase

genes are expressed in Arabidopsis, to the best of our knowledge the presence of

functional β-galactosidase enzyme has not yet been verified. Although it is desirable to

carry out detailed biochemical characterizations using native enzymes, the isolation and

purification of enzymes from native sources can be challenging. We therefore focused on

achieving this goal for a representative member of Subfamily III. A combination of

affinity, hydrophobic interaction, and ion-exchange chromatography was used to isolate

and biochemically characterize a native β-galactosidase protein from mature rosette

leaves, the tissue that can be most easily collected in large quantities from Arabidopsis.

Purification was monitored using a β-galactosidase activity assay, SDS-PAGE, and

immunoblotting until a single protein band was obtained (Fig. 4). MALDI-TOF analysis

after in-gel trypsin digestion gave a peptide mass fingerprint consistent with that of Gal-2

(Fig. 4C). We therefore designated the preparation as native Gal-2 (nGal-2). This is the

first successful isolation of a native β-galactosidase protein from Arabidopsis.

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Fig. 4. SDS-PAGE (A), native PAGE (B), and MALDI-TOF spectrum (C) of purified

native Gal-2. (A) 10% SDS-PAGE gel stained with Coomassie Blue R-250 (Lane 1,

molecular weight standard; Lane 2, purified native Gal-2); (B) 8% acidic native gel

81

The biochemical properties and substrate specificities of nGal-2 were studied and

compared with those of the recombinant enzyme. nGal-2 showed a monomeric molecular

weight of ~75kD by SDS-PAGE. This is consistent with the size predicted for this

protein, indicating that the nGal-2 polypeptide is intact and that it does not undergo

proteolytic processing that has been reported for other plant β-galactosidases (Kotake et

al., 2005; Ross et al., 1994; Triantafillidou and Georgatsos, 2001). The size of the protein,

together with the fact that it did not bind to ConA (not shown), indicated that it was not

glycosylated. This is also consistent with our observations that recombinant Gal-2

expressed in P. pastoris is also not glycosylated (Gantulga et al., 2008). nGal-2 has a

basic pI that allows the enzyme to migrate into an acidic native gel, showed a strict sugar

specificity toward galactose, with some activity towards fucose, and hydrolyzed the

artificial substrates, pNPGal, oNPGal, X-Gal, and pNFuc (Table I, Table II, and Fig. 4).

The pH optimum of the enzyme was ~4.0, which is typical for plant β-galactosidases

(Buckeridge and Reid, 1994; Carey et al., 1995; Kotake et al., 2005; Li et al., 2001; Ross

et al., 1994). Hydrolysis of pNPGal by nGal-2 showed typical Michaelis-Menten kinetics

(Table II). The linkage specificity of nGal-2 was the same as that of the recombinant Gal-

2, in that it preferred to cleave β-(1, 4) and β-(1, 3) linkages in galacto-oligosaccharides

(data not shown). nGal-2 also hydrolyzed apple pectin and ammonium oxalate-soluble

pectic fractions of Arabidopsis cell walls, releasing monomeric galactose, similar to the

recombinant enzyme (Table IV).

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2.6 Localization of β-galactosidase activity in whole tissues

To further explore the potential roles of the Subfamily III enzymes, an effort was

made to examine the tissue-specific distribution and subcellular localization of the

isozymes. As an initial approach, X-Gal was used as a substrate at pH4.6 to visualize β-

galactosidase activity in 14-day-old Arabidopsis seedlings, as well as other tissues from 5-

week-old plants. Control tissues incubated with X-Gal in the presence of galactone-

lactone (a galactosidase inhibitor) or glucono-lactone (a glucosidase inhibitor) or with X-

Glu (a glucosidase substrate) at pH 4.6 did not show any staining (not shown). This

indicates that any observable staining resulted only from β-galactosidases with acidic pH

optima. β-galactosidase activity was present primarily in vascular tissues of young leaves

(Fig 5A) and flowers (Fig. 5B and C), as well as in the silique and in seeds (Fig. 5D and

E). The intense staining observed in the anther and stigma (Fig. 5B and C) is consistent

with a role for β-galactosidases in pollen development and germination. In root, β-

galactosidase activity was restricted to the elongation zones, and no staining was detected

in the root tip (not shown).

2.7 Organ specific expression of Gal-1 and Gal-12: Immunoblotting using anti-peptide

antibodies

Isoform-specific antipeptide antibodies were prepared for the two enzymes with

the highest (Gal-1) and lowest (Gal-12) levels of expression from microarray data among

the members of Subfamily III. The antibody preparations were affinity-purified and then

used to detect the relative abundance and size of the native Gal-1 and Gal-12 proteins in

Arabidopsis root, petiole, rosette leaf, stem, cauline leaf, flower, and silique (Fig. 6). Gal-

83

Fig. 5. Localization of β-galactosidase activity in Arabidopsis whole tissues. Tissues

from four-week-old plants were stained with X-Gal. (A) cotyledon; (B) flower; (C)

stamen; (D) silique; (E) seed. The arrow in D indicates the location of the seed shown in

E.

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Fig. 6 Immunoblot analysis of organ-specific expression of Gal-12 and Gal-1.

Arabidopsis total protein extracts (10 µg) were separated on a 10% SDS–PAGE gel. The

proteins were either transferred to PVDF membranes and incubated with affinity purified

rabbit antibodies against Gal-12 and Gal-1 peptides (A) or stained with Coomassie blue R-

250 (B). The arrows in panel B mark the approximate size and positions of the

immunoreactive bands in panel A.

85

12 protein was detected as a single band of approximately 75 kD in all organs, consistent

with the predicted molecular mass for this enzyme, indicating that native Gal-12 is not

glycosylated, as was also observed for the protein produced in P. pastoris, and that it is

not proteolytically processed. The relative abundance of Gal-12 protein was highest in the

root, moderate in flowers, and low in all other organs. We observed discrepancies in the

the relative abundance of the transcript and protein in some cases. This could be due to

the differences in transcriptional and translational regulation, as proposed previously for

Gal-5 (Gantulga et al., 2008).

The anti-peptide antibody for Gal-1 reacted with two bands of ~50 and ~60 kD in

all tissues (Fig. 6). Unlike Gal-2 and Gal-5 (Gantulga et al., 2008) and the Gal-12 protein,

Gal-1 protein appeared to be post-translationally processed into fragments much smaller

than the predicted polypeptide size of 90 kD. It should be noted that proteolytic

processing has also been observed for β-galactosidases in apple (Ross et al., 1994), rice

(Chantarangsee et al., 2007; Kaneko and Kobayashi, 2003), barley (Triantafillidou and

Georgatsos, 2001), and radish (Kotake et al., 2005; Sekimata et al., 1989). Gal-1 protein

was most abundant in root and flower, which is consistent with the microarray and RT-

PCR data (Figs. 2 and 3), but as with Gal-12 the mRNA and protein expression patterns

are not exactly the same, pointing to possible posttranscriptional or posttranslational

regulation.

The overall expression patterns of Gal-1 and Gal-12 proteins are similar to that of

of Gal-2, with accumulation of the protein in most organs, while Gal-5 protein is found

only in green tissues (Gantulga et al., 2008). Both Gal-1 and Gal-12 are most abundant in

the root, suggesting possible cooperation between the enzymes in growth control in this

86

organ. Specific antibodies will have to be developed against Gal-3 and Gal-4 in order to

extend this analysis to the entire subfamily.

2.8 Immunolocalization of Gal-1 and Gal-12

Although there is good evidence for a role for β-galactosidases in cell wall

modification, the subcellular localization of the enzymes in plant has not yet been

determined at high resolution. The anti-peptide antibodies developed for Gal-1 and Gal-

12 were therefore used to localize these proteins by immuno-electron microscopy.

According to the immunoblotting and total β-galactosidase activity-staining experiments

(Figs. 5 and 6), Gal-1 and Gal-12 are likely to be most strongly expressed in the

elongation zone of the mature root. The localization experiments were therefore

performed using root tissue. A light microscopy image of a transverse section of an

unlabelled root tissue is shown in Fig. 7A. Fig. 7B shows an enlargement of a xylem cell

shown in Fig. 7A by transmission electron microscopy (TEM). The sections from the

same tissue, with similar morphology as in Fig. 7B, were then labeled either with purified

anti-Gal-12 or with anti-Gal-1 antibody, and then with a gold-conjugated secondary

antibody.

Transmission electron microscopy images revealed that Gal-12 and Gal-1 proteins

are localized in thickened walls of xylem cells. For Gal-12, the areas near the pit of xylem

vessels were labeled more intensely, Fig. 7C. Less intense labeling of Gal-12 was

observed in the cell walls of the epidermal cells and root hairs. In the case of Gal-1, only

xylem cell wall was labeled; no gold particles were observed in any other areas of the root

section. Unlike Gal-12, which often appeared as clusters of particles inside and around

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Fig. 7 Subcellular localization of Gal-1 and Gal-12. (A) Light microscopy image of an

unlabeled toluidine-blue stained transverse section of the root. (B) TEM image of the

xylem cell. (C) Gal-12 labeled xylem cell wall. (D) Gal-1 labeled xylem cell wall. “x”-

xylem cell; “c”-cortex, “en”-endodermis; and “cw”-cell wall. Arrows indicate locations of

the gold particles.

88

small, electron dense areas, Gal-1 labels appeared as single particles scattered and evenly

distributed along the cell wall. Control sections incubated with a buffer in place of the

primary antibodies showed no labeling, indicating the labeling of Gal-1 and Gal-12 in Fig.

7C-D was specific. Localization of β-galactosidase proteins in the thickened areas of the

root cell wall provides strong evidence that the enzymes encoding the Subfamily III genes

are involved in the degradation of cell wall polysaccharides.

2.9 Conclusions

Here we have described the characterization of four recombinant proteins

expressed in P. pastoris, completing the systematic analysis of the Subfamily III β-

galactosidases of Arabidopsis. A native protein from the subfamily was also purified and

shown to have biochemical properties similar to those of the recombinant protein. All of

the enzymes were found to have a strict galactose specificity and a broad aglycone

specificity. The enzymes all cleave β-(1, 4) and β-(1, 3) linked galacto-oligosaccharides,

while only Gal-12 hydrolyzed β-(1, 6) linked galacto-oligosaccharides. All of the

enzymes also have activity toward pectic polysaccharides, releasing monomeric galactose

as a product. This indicates that Subfamily III β-galactosidases are exo-galactanases that

primarily act on galactan side chains of cell wall polysaccharides. It will be interesting to

examine these differences at the structural level. Unfortunately, although structures have

been solved for three GH35 enzymes, these are from microbial species that do not have

sufficient similarity to the plant enzymes to allow for reliable homology modeling.

Spatial expression of six Gal genes was studied by RT-PCR, which revealed that

the Gal genes are differentially regulated. Methyl jasmonate known to regulate growth

and defense response in plants down-regulates expression of at least three out of six Gal

89

genes. Isoform-specific antibodies showed that levels of the Gal-1 and Gal-12 proteins

were high in root, moderate in flower, and low in all other organs examined. While the

Gal-12 polypeptide in plant extracts was similar to the predicted size, Gal-1 appears to

undergo posttranslational cleavage to two smaller fragments. We also observed numerous

discrepancies between mRNA and protein levels which suggests possible transcriptional

and/or post-transcriptional regulation of Gal gene expression.

To the best of our knowledge, we show for the first time the cell wall localization

of β-galactosidases by TEM. Mature root sections labeled with Gal-12 and Gal-1

antibodies showed specific labeling of cell wall of vascular and epidermal tissues. More

detailed study is necessary to establish whether Gal-1 and Gal-12 labeling corresponds to

regions where β-(1, 4)-galactan accumulates, suggested to be associated with elongation

and differentiation of root cells (Martin et al., 2008; McCartney et al., 2003). It will also

be interesting to study the localization of Gal-1 and Gal-12, as well as other Subfamily III

Gals, in tissues of other organs. Our preliminary study revealed labeling in the cell wall of

rosette leaf and flower for Gal-1 and Gal-2 (data not shown). It may be possible to derive

more direct insights into the physiological functions of these enzymes through genetic

analysis. However, this may require construction of double or triple mutants or RNAi

lines because preliminary characterization of T-DNA insertion lines for Gal-1, Gal-3, and

Gal-12 uncovered no obvious phenotypes (data not shown; Nickolas Carpita, Perdue

University, personal communication)..

One of the central goals of the NSF Arabidopsis-2010 project is to characterize all

putative genes in this model plant. As for seventeen β-galactosidases in GH Family 35,

seven have been functionally characterized to date. In addition to the six enzymes that

90

belong to Subfamily III (this report and Ahn et al., 2007; Gantulga et al., 2008), Gal-6

from Subfamily V has been characterized by Dean et al. (2007). Clearly, much remains to

be learned about the functions of the remaining members of the large family and whether

the subfamilies reflect functional redundancy of the isozymes.

3. Experimental

3.1 Plant material

Arabidopsis Col-O seeds were obtained from the Arabidopsis Biological Resource Center

(Columbus, OH). Plants were grown as described previously (Gantulga et al., 2008). For

stress and hormone treatments, 14-day old seedlings on plates were sprayed directly. At

different time points after treatment, seedlings were harvested, snap frozen in liquid

nitrogen and kept at -80oC until RNA isolation. Cell wall isolation and extraction of cell

wall polysaccharides were carried out as described in Li et al (2001). For immunoblot

analysis, Arabidopsis tissue was ground in 1X SDS Laemmli sample buffer containing 6M

urea at a ratio of 1:4 (0.5 g tissue: 2 ml solvent).

3.2 Expression and purification of recombinant Gal-1, Gal-3, Gal-4, and Gal-12 in P.

pastoris

The mature protein coding sequences of Gal-1, amino acids Ser33 to Trp815, Gal-

3, residues Gly32 to Pro825, Gal-4, residues Ser28 to Thr697, and Gal-12, residues Ile28

to Lys701, were cloned into the pPICZα expression vector (Invitrogen, Carlsbad, CA) for

expression fused to the C-terminus of the yeast α factor secretion signal to facilitate

91

secretion of recombinant proteins into the culture medium. Expression of recombinant

Gal-4 and Gal-12 proteins was carried out following the manufacturer’s protocol. A

modified protocol was used for expression of Gal-1 and Gal-3, as follows. Initial biomass

was obtained by growing 1L-1.5L culture for 2-3 days, then transferring the cells into 100

ml of induction medium to initiate expression of the recombinant proteins. Supernatant

fractions from seven-day old cultures were used for purification of the enzymes.

Gal-1 and Gal-4 were purified from culture supernatants by ion exchange

chromatography using Sulfoxyethyl (SE 53) cellulose (Whatman, UK). One hundred ml

of culture supernatant was filtered, diluted five times with degassed dH2O to reduce the

ionic strength of the medium and loaded onto the column pre-equilibrated with buffer A

(20 mM potassium phosphate, pH 6.0). After washing with 10 column volumes of buffer

A, bound proteins were eluted in one step with 200 mM NaCl in buffer A. All fractions

were checked for β-galactosidase activity using pNPGal as substrate. Fractions with the

highest β-galactosidase activity were used for further experiments.

Gal-3 and Gal-12 proteins were purified using hydrophobic interaction

chromatography and ion exchange chromatography. Solid ammonium sulfate was added

to the culture supernatant to 30% final concentration, and filtered supernatant was loaded

onto a ToyoPearl Butyl column (Tosoh Bioscience, Montgomeryville, PA) pre-

equilibrated with 25 mM potassium phosphate buffer, pH 6.0 with 30% ammonium

sulfate. After washing the column with 5 volumes of equilibration buffer, bound proteins

were eluted with a linear 30-0% ammonium sulfate gradient in 25 mM potassium

phosphate buffer, pH 6.0. Active fractions were pooled and desalted using a 30 kD cutoff

centrifugal concentrator (Vivascience, UK) with three changes of 10 mM sodium acetate

92

buffer pH 5.0. The sample was then loaded onto a cation-exchange Accell-CM column

(Waters, Milford, MA) pre-equilibrated with desalting buffer and eluted with a 0-300 mM

NaCl gradient. Active fractions were pooled and concentrated using a centrifugal filter

and stored on ice at 4oC for further use.

3.3 Purification of native Gal-2 from Arabidopsis rosette leaves

Fresh rosette leaves (100 g) from 4-week-old Arabidopsis plants were

homogenized in 500 ml extraction buffer (EB buffer, 10 mM potassium phosphate, pH

7.0, 10 mM EDTA, 1 mM MgCl2, 5% glycerol, 1 mM PMSF). The resulting homogenate

was incubated on ice for 2 h and centrifuged at 12000 x g for 20 min at 4oC. The pellets

were re-extracted with EB and centrifuged. The two supernatants were combined and

filtered. The crude extract was loaded onto a Concanavalin-A (ConA) agarose column

(Vector Laboratories, Burlingame, CA) equilibrated with ConA buffer (10 mM TRIS-HCl,

pH 7.0, 1 mM CaCl2, 500 mM NaCl). Pass-through fractions were collected, solid

ammonium sulfate (AS) was added to 25% final concentration, and the solution was

incubated at 4oC for 1 h. Precipitated proteins were removed by centrifugation and the

supernatant was loaded onto a ToyoPearl Butyl (TPB) column, equilibrated with TPB

buffer (20 mM potassium phosphate, pH 6.2, 25% AS). After washing the column with

50 ml of TPB buffer (5 bed volumes), bound proteins were eluted with a linear gradient of

AS from 25-0% in 20 mM potassium phosphate, pH 6.2. All fractions were checked for β-

galactosidase activity using pNPGal as a substrate. Two peaks were detected. The

fractions from the later-eluting peak were pooled and loaded onto the Sulfoxyethyl

cellulose (SE) (Whatman, UK) equilibrated with SE buffer (20 mM potassium phosphate,

93

pH 6.2) after a 10-fold dilution with SE buffer. All β-galactosidase activity was bound to

the column. Bound fractions were eluted with a linear 0-500 mM NaCl gradient in SE

buffer. A single activity peak was detected using pNPGal as a substrate. Active fractions

were pooled and concentrated on a small SE column. Purified protein was subjected to

MALDI-TOF/MS after trypsin digestion. Peptide fragment masses were analyzed using

MASCOT at http://www.matrixscience.com.

3.4 β-galactosidase activity assay

The standard protocol described in Gantulga et al. (2008) was used for all activity

assays with pNPGal, unless stated otherwise. The hemagglutination assay for lectin

activity was carried out as described in Kittur et al. (2007).

In order to determine natural substrate specificities, galacto-oligosaccharides (20

mM), xyloglucan olisaccharides (XGOs) (1 mg/ml), and polysaccharides (1% (w/v)) were

prepared in water. Final concentrations were 4 mM for oligosaccharides, 0.5 mg/ml for

XGOs, and 0.5% for polysaccharides in 100 mM acetate buffer pH 4.6. Reaction mixtures

were incubated with 0.04 units/nkat of enzyme at room temperature for 30 h. Reaction

mixtures containing no enzyme and no substrate were used as controls. Quantification of

galactose produced after hydrolysis of polysaccharides by β-galactosidases was carried out

as described in Gantulga et al. (2008). Hydrolysis of XGOs was analyzed by MALDI-

TOF as described in Iglesias et al. (2006).

3.5 Localization of β-galactosidase activity in Arabidopsis whole tissues

94

Ten-day-old seedlings and tissues from 4-week-old plants were washed twice with

dH2O and incubated with staining solution (50 mM sodium acetate buffer pH 4.6, 10 mM

EDTA, 0.1% sarkosyl, 20% methanol, 1 mM X-Gal (5-bromo-3-indolyl-β-D-

galactopyranoside)) at 37oC for 2h. Tissues were then de-stained in methanol 3 times for

1h for each until the indigo staining was visible (Chantarangsee et al., 2007).

3.6 RT-PCR analysis

Total RNA was extracted from 100 mg of seedling and from mature plant tissues

using Trizol reagent (Invitrogen, Carlsbad, CA) and treated with Turbo RNase free DNase

(Ambion, Foster City, CA) to remove contaminating genomic DNA. cDNA was prepared

from these RNA samples (2-5 µg) using the SuperScript II reverse transcriptase kit

(Invitrogen, Carlsbad, CA) and oligo(dT) primers. The reverse transcription reaction was

diluted 100-300 times with nuclease-free water and used for PCR analysis of gene

expression using GoTaq Green mix (Promega, Madison, WI). Ubiquitin-10 (At4g05320)

expression was analyzed as a control.

3.7 Generation of peptide-specific antibodies

For generating isozyme specific antibodies, unique peptides

CSQTSRMKMVRVPVHGGL and CSPDEKFLTGEDPLLTIG derived from Gal-1 and

Gal-12 polypeptide sequences, respectively, were designed. Each peptide sequence was

compared against the Arabidopsis genome sequence using tblastn (blast.ncbi.nlm.nih.gov)

to confirm that these represented unique epitopes. Peptide synthesis and generation of

antibodies were carried out using the Genscript antibody service (Piscataway, NJ). The

95

isozyme-specific antibodies were purified using antigen-affinity columns, prepared by

coupling Gal-1 synthetic peptide or purified recombinant Gal-12 protein to AffiGel-10

(Bio-Rad, Hercules, CA) according to the manufacturer’s protocol. The specificity of the

antibodies was verified by probing dot blots containing recombinant protein for all six

Subfamily III members under the conditions used for the immunoblot analysis of

Arabidopsis tissues (data not shown).

3.8 SDS-PAGE, native PAGE and immunoblotting

SDS-PAGE was performed as described by Laemmli (1970). Immunoblots were

carried out as described in Gantulga et al. (2008) except that detection was by

chemiluminescence using the ELC-Plus kit (GE Healthcare, Piscataway, NJ) and CL–

Exposure film (Thermo, Rockford, IL).

3.9 Immunolocalization

For immunolocalization studies, Arabidopsis root samples were prepared as

described in Saslowsky et al. (2001). Thin sections were collected on Formvar-coated

nickel grids. For TEM analysis, ultra-thin LR White sections were first incubated with

100 mM glycine pH 7.0 to block residual aldehyde groups. Then grids were transferred to

10% Triton-X and incubated for 15 min. Nonspecific binding sites were blocked with

10% (w/v) normal goat serum (Sigma, St. Louis, MO) in TBS for 1 h at room temperature.

Grids were incubated at room temperature with affinity purified anti-Gal-1 antibody

diluted 1:2 and anti-Gal-12 diluted 1:5 in TBS containing 10% (w/v) normal goat serum

overnight at 4oC. After three 10 min washes in TBST, grids were incubated with 10 nm

96

gold-conjugated goat anti-rabbit antibody (Sigma, St. Louis, MO) diluted 1:100 for Gal-12

and 1:50 for Gal-1 in 10% (w/v) normal goat serum in TBS for 1 h at room temperature.

Grids were washed three times in TBST for 10 min and three times in water for 10 min.

Sections were counterstained with 2% (w/v) uranyl acetate for 15 min and washed in

distilled water. As a control, primary antibodies were replaced with buffer. Immunogold-

labeled sections were examined with a JEOL (Tokyo, Japan) JEM 100CX-II transmission

electron microscope. Thick sections of the same samples were collected on glass slides,

stained with toluidine blue, mounted on permount, and examined by light microscopy.

Acknowledgements

We are grateful to Drs. Yoichi Tsumuraya and Dr. Toshihisa Kotake of Saitama

University, Japan for providing galacto-oligosaccharides. The authors also wish to thank

Drs. R.A Walker, D. Cimini and J. Sible for the light microscopy analysis and K. Lowe

for the Tem analysis. This research is funded by the Arabidopsis 2010 Project of the

National Science Foundation (MCB-0115937) to Esen et al., a research grant award from

the Virginia Academy of Science to D. Gantulga and Department of Biological Sciences,

VT.

97

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103

Chapter 4

Conclusion

104

Conclusion

The Arabidopsis β-galactosidase GH35 Subfamily III, consisting of six members,

was systematically characterized with respect to enzyme activity, substrate specificity,

organ- and tissue-specific expression constituting a first complete characterization of the

entire subfamily from a single species (Ahn et al., 2007; Gantulga et al., 2008a; Gantulga

et al., 2008b) . Based on identification of natural substrates and subcellular localization,

we conclude that these β-galactosidases are most likely involved in cell wall modification

during plant growth and development.

All six members of Subfamily III were expressed in Pichia pastoris as

recombinant proteins and characterized with a special emphasis on substrate specificities.

A native β-galactosidase purified from Arabidopsis rosette leaves displayed biochemical

characteristics similar to those of the recombinant enzyme, suggesting that the properties

of the recombinant proteins accurately reflect those of the native counterparts. The

natural substrate specificities of these six isoforms suggest that Subfamily III β-

galactosidases are exo-galactanases that primarily act on galactan side chains of cell wall

polysaccharides.

Spatial expression of the six Gal genes was studied by analysis of publicly

available microarray data and by RT-PCR experiments, revealing different expression

patterns in different organs. Methyl jasmonate, which is known to regulate plant growth

and defense, was found to down regulate the expression of at least three Gal genes.

Peptide-specific antibodies were generated against four isoforms, Gal-1, Gal-2, Gal-5,

and Gal-12, and purified using antigen-affinity columns. These antibodies were used to

105

study the expression of these four isoforms at the protein level by immunoblotting. The

organ-specific expression patterns were generally consistent with microarray and RT-

PCR data, although some discrepancies were observed between relative abundances of

the transcripts and proteins. This suggests that there are distinct mechanisms of regulation

for transcription and translation. The sizes of the Gal-2, Gal-5 and Gal-12 polypeptides in

plant extracts were similar to the predicted sizes for these enzymes, whereas Gal-1

appeared to undergo a posttranslational cleavage to two smaller fragments, similar to

what has been observed for β-galactosidases in apple (Ross et al., 1994), radish (Kotake

et al., 2005), and barley (Triantafillidou and Georgatsos, 2001).

To explore the localization of the β-galactosidase proteins, enzyme activity was

first visualized in vivo using the substrate, X-Gal. β-galactosidase activity was present

mostly in vascular tissues of young leaves, flowers, and roots, as well as in seed. Cell

wall localization of Gal-1, Gal-2, Gal-5, and Gal-12 was shown by immuno-dot blotting

and western blotting of apoplastic and cell wall bound protein extracts. Subcellular

localization of β-galactosidase proteins was further studied by immunofluorescence and

immuno-electron microscopy in organs where β-galactosidase activity was abundant

based on the staining experiment. In particular, immunogold labeled particles for Gal-1

and Gal-12 proteins were detected in the cell wall of vascular and epidermal tissues in

mature root. To the best of our knowledge, this is the first time that the cell wall

localization of β-galactosidases has been shown at high resolution by transmission

electron microscopy. Localization of the β-galactosidase proteins in the cell wall and the

identified natural substrate suggests that these proteins have a potential to modify cell

wall structure.

106

Determination of functions and characteristics of the Subfamily III genes is a

valuable contribution to the NSF Arabidopsis 2010 project, whose central goal is to

determine the functions of all 24,000 predicted genes in this important model plant. The

systematic approach taken in this work, from biochemical characterization to subcellular

localization through expression patterns, can be successfully applied in determining the

functions of the remaining Arabidopsis β-galactosidases as well as many other genes.

The results presented in this dissertation can be helpful in further studies aimed at

understanding the metabolic and gene regulatory networks in which β-galactosidases

participate to modify cell wall structures.

Finally, as for further research on Subfamily III β-galactosidases, interesting goals

are to increase catalytic efficiency and alter substrate specificity of the enzymes by site-

directed mutagenesis. Mutant genes encoding highly efficient and controllable β-

galactosidases can be inserted into the genome of different plants for specific

modification of cell wall structures. Such transgenic plants may have improved

agronomic qualities, enhanced defenses against herbivores, drought tolerance, and long

term post-harvest shelf life.

107

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clone. Plant Physiol. 106, 521-528.

Triantafillidou, D., Georgatsos, J. G., 2001. Barley β-galactosidase: Structure, function,

heterogeneity, and gene origin. J. Protein Chem. 20, 551-562.